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Full text of "Anatomy and histology of the mouth and teeth"




A GIFT FROM 



Dr. F. M. Farnsworth 



Buckhannon, W. Va, 
1956 








A?z 



DO NOT CIRCULATE 



ANATOMY AND HISTOLOGY 

OF THE 

MOUTH AND TEETH 



BROOMELL AND FISCHELIS 



ANATOMY AND HISTOLOGY 



OF THE 



MOUTH AND TEETH 



BY 

I. NORMAN BROOMELL, D. D. S. 

DEAN AND PROFESSOR OF PROSTHETIC DENTISTRY, DENTAL ANATOMY AND HISTOLOGY, DENTAL 
DEPARTMENT, MEDICO-CHIRURGICAL COLLEGE OF PHILADELPHIA 

AND 

PHILIPP FISCHELIS, M. D. 

ASSOCIATE PROFESSOR OF HISTOLOGY AND DEMONSTRATOR IN EMBRYOLOGY, MEDICO-CHIRURGICAL 

COLLEGE OF PHILADELPHIA 



THIRD EDITION, REVISED, WITH 357 ILLUSTRATIONS 



PHILADELPHIA 

BLAKISTON'S SON & CO 

1012 WALN.UT STREET 
1910 



U3RARY 
MENTAL SCHOOL 
W.V.U. 



OLD BOOKS 

Copyright, 1909, By P. Blakiston's Son & Co. 



Printed by 

The Maple Press 

York, Pa. 



TO 
C. N. PEIRCE, D. D. S. 

AS A SOUVENIR OF A LONG AND VALUED FRIENDSHIP AND A 

TESTIMONY OF ESTEEM FOR HIS PROFESSIONAL 

AND PRIVATE WORTH 

THIS VOLUME IS RESPECTFULLY DEDICATED 

BY THE AUTHORS 



PREFACE TO THIRD EDITION. 



In submitting to the dental profession a revised edition of this work, 
the author has embraced the opportunity to make such corrections and 
additions as were deemed advisable to further increase the value of the 
book and to make it worthy of at least a portion of the approval accorded 
the first edition. The general plan of the work has not been altered, and 
while some exceptions have been taken to including within its pages 
chapters devoted to the general anatomy of the mouth and its associated 
parts, it was decided, after due consideration, to allow this to remain. 

In the description of the teeth, the terms "superior" and "inferior" 
have been changed to "upper" and "lower" and the term "palatal" 
as applied to one of the tooth surfaces has been. discarded, and the word 
"lingual" substituted. 

Substantial additions have been made to Part II (Chapters I, II, 
VII, etc.). It has been justly brought forward within recent years* that 
there is a lack of interest and enthusiasm for collateral reading, special 
studies, and original investigation not only among students, but also 
among graduates of Dentistry. The experience gained in teaching has 
suggested to the authors that this is due to the fact that dental students 
use to a great extent text-books on Anatomy, Histology, and Embryology, 
chiefly intended for medical students, and they generally believe that 
most of the matter which is contained in these books is not intended for 
them, having no practical application. Their attention is therefore 
limited to those chapters which seem of importance to them, and thus an 
intelligent understanding of the subject is not obtained. Recognizing 
this fact, it seemed advisable to include in this Dental Text-book a de- 
scription pertaining to general Cytology, general Embryology, and 
Histogenesis, and a further presentaJ'crbr lie s»g nltlca 0IT i'ce of the facts thus 
gained for an intelligent understanding of the f°*^imation of the organs of 
the mouth. 

It has been the aim to present the subject in a concise and attractive 
form, including some illustrations which never before appeared in a 
work designed for dental students. It is the hope of the authors that 
the additions made will help to awaken an interest for collateral reading 
and thus elevate the standard of education among dental graduates. 
Philadelphia, November 15, 1909. 

* Proceedings of the Meeting of the Institute of Dental Pedagogics. 

vii 



PREFACE TO THE FIRST EDITION 



In the preparation of this work it has been the aim of the author to 
systematically describe those parts of human anatomy which come 
directly under the care of the stomatologist. In the earlier chapters, 
which are devoted to a gross description of the mouth and those tissues 
which enter into its construction, there has been no attempt at originality 
other than in the arrangement, which includes a complete description of 
one part before another is taken up. 

In the writing and classification of the succeeding chapters the writer 
has attempted what others, though wiser and better qualified, appeared 
unwilling to undertake, and it is from the works of such as these that the 
foundation for the present work has been derived. 

Within the last few years the progress in nearly every branch of 
dental education has made a work of this character an imperative want. 
Dental therapeutics and dental chemistry have been well-nigh recon- 
structed, while the investigations of the microscopist and physiologist have 
brought forth many valuable revelations. Next in importance has been 
the advance in, or rather the introduction of, technic teaching. Con- 
siderable space has, therefore, been devoted to the surface anatomy of the 
individual teeth, with a hope that it may be of value in dental anatomy 
technic. 

While in one or two instances the writer has departed from the field 
assigned as a text, the parts thus included are so closely associated with 
the mouth, both in a constructive and in a functional manner, that the 
work would be lacking in completeness if they were omitted. 

The illustrations are, with but few exceptions, the original work of 
the author, being reproduced by photograph from the actual subject. 
In many instances dissections were required to reveal the parts, this 
being particularly true of those illustrations included in the chapter on 
the Development of the Teeth, about one hundred dissections being re- 
quired to accomplish the purpose. In preparing the illustrations de- 
scriptive of the various surfaces of the individual teeth, the progress of 
the work was materially interfered with by the difficulty experienced in 
securing normal teeth out of the mouth; may their number ever grow less. 

ix 



X PREFACE TO THE FIRST EDITION. 

The author desires to thus publicly acknowledge obligations to the 
works of Tomes, Black, Morris, Stohr, Klein, and Strieker. He is also 
indebted to Prof. A. P. Brubaker and to Dr. C. P. Shoemaker for valuable 
assistance rendered, and to P. Blakiston's Son & Co. for their many cour- 
tesies during the preparation of the volume. 

That there is a place for such a work as this purports to be the writer 
has but little doubt; that the following pages will fill that demand is his 
earnest desire, and it remains for the reader to ascertain how far these 
demands have been met in the direction of its aim and endeavor. 






TABLE OF CONTENTS. 



PART I.— ANATOMY. 

CHAPTER I. 

General Description of the Mouth. — The Buccal Orifice; The 
Lateral Walls of the Mouth; The Hard Palate, or Dome of 
the Mouth; The Soft Palate and Fauces; The Floor of the 
Mouth; The Tongue and its Attached Muscles, i 

CHAPTER II. 

The Bones of the Mouth. — The Superior Maxillae; The Palate 

Bones; The Inferior Maxilla, or Mandible, 34 

CHAPTER III. 

The Temporomandibular Articulation. — The Muscles of Masti- 
cation, 61 

CHAPTER IV. 

General Description of the Teeth; The Permanent Teeth; Classi- 
fication, Surfaces, etc.; The Roots of the Teeth; The Dental 
Arch, 72 

CHAPTER V. 
Occlusion of the Teeth, 85. 

CHAPTER VI. 
The Blood- and Nerve-supply to the Teeth, 91 

CHAPTER VII. 

Other Structures Within the Mouth.— The Gums; The Mucous 
Membrane; The Alveolodental Membrane; Glands, Ducts, 

etc., 99 

xi 



Xll TABLE OF CONTENTS. 

CHAPTER VIII. 

A Description of the Upper Teeth in Detail. — Calcification, Erup- 
tion, and Average Measurements; Their Surfaces, Ridges, 
Fossae, Grooves, etc., no 

CHAPTER IX. 

A Description of the Lower Teeth in Detail. — Calcification, Erup- 
tion, and Average Measurements; Their Surfaces, Ridges, 
Fossae, Grooves, etc., 172 

CHAPTER X. 

The Pulp-cavities of the Teeth, 198 

CHAPTER XL 

The Deciduous Teeth, Their Arrangement, Occlusion, etc.; Cal- 
cification, Eruption, Decalcification, Shedding Process, and 
Average Measurements; Their Surfaces, Grooves, Fossae, 
Ridges, and Pulp-cavities, 224 

CHAPTER XII. 

Development of the Teeth. — The Dental Germs, Enamel Organ, 
and Dentin Organ; The Dental Follicle; Calcification, Erup- 
tion, etc., 240 



PART II.— HISTOLOGY AND HISTOGENESIS. 

CHAPTER I. 
General Cytology; General Embryology and Histogenesis, . . . 293 

CHAPTER II. 

Elementary Tissues. — -Epithelial Tissue; Connective Tissue; 

Muscular Tissue; Nervous Tissue; Blood and Lymph, . . 308 

CHAPTER III. 

The Mucous Membrane of the Mouth. — Of the Lips; Of the 
Cheeks; Of the Gums; Of the Roof of the Mouth, Hard and 
Soft Palate; Of the Floor of the Mouth and the Tongue, . . 328 



f> ' TABLE OF CONTENTS. xiii 

CHAPTER IV. 

Glands and Ducts of the Mouth. — Of the Lips; Of the Cheeks; 
Of the Hard and Soft Palate; Of the Tongue; The Salivary 
Glands, 339 



CHAPTER V. 

Muscular Tissues of the Mouth. — Of the Lips; Of the Cheeks; 

Of the Soft Palate; Of the Tongue, 347 

CHAPTER VI. 

Tissues of the Teeth. — Enamel; Dentin; Cementum; The Tooth- 
pulp; The Alveolodental Membrane, 350 

CHAPTER VII. 

Embryology of the Mouth and Teeth, 411 

CHAPTER VIII. 
Anomalies of the Teeth, 437 

Index, 447 



ANATOMY AND HISTOLOGY 



OF THE 



MOUTH AND TEETH 



PART I. -ANATOMY. 



CHAPTER I. 

General Description of the Mouth. — The Buccal Orifice (the Lips). — 
The Lateral Walls of the Mouth (the Cheeks).— The Hard 
Palate or Dome of the Mouth.— The Soft Palate and Fauces. 
—The Floor of the Mouth.— The Tongue and its Attached 
Muscles. 

The mouth (Fig. i) (stoma, pi. stomata) is the entrance or gateway 
to the alimentary canal, and is situated between the superior and inferior 
maxillary bones and their attached tissues. It contains the active organs 
of mastication, the teeth, the organs of taste, of which the tongue is chief, 
together with some of the parts which assist in articulate speech. Anato- 
mists usually divide this cavity into two compartments, the teeth serving 
to separate one from the other, the inner space being called the mouth, 
while that between the teeth and lips or cheeks is known as the vestibule 
of the mouth. In this description all that space bounded anteriorly by the 
lips, posteriorly by the pillars of the fauces, and latterly by the cheeks, 
will be considered as a single cavity, and the organs and structures con- 
tained therein, together with all parts directly interested in its formation, 
will constitute a text for this work. The entrance to the cavity of the 
mouth is formed by a freely movable transverse orifice or slit, the buccal 
orifice, while communication is made with the pharynx posteriorly 
through the fauces. Entering into the construction of the mouth and 
assisting in the performance of its functions are bones, ligaments, muscles, 
blood-vessels, nerves, glands, ducts, etc., each of which will be described 
in turn. 



ANATOMY 



THE BUCCAL ORIFICE, 

or entrance to the cavity of the mouth, is a transverse opening somewhat 
variable in extent, the extremities of which are known as the corners or 
angles of the mouth. The orifice is bounded by two fleshy folds, the 




Fig. i. — Outer Wall of Nasal Fossa, with Mouth, Pharynx and Larynx in Vertical 

Section. (Dearer.) 

a, Superior meatus; b, superior turbinate body; c, middle turbinate; d, inferior turbinate; 
e, inferior meatus; g, tongue; h, posterior pillar of fauces; i, geniohyogtossus muscle; ;', geni- 
ohyoid muscle; k, hvoid bone; I, mylohyoid muscle; m, thyrohyoid membrane; n, ventricle 
of larynx; o, thyroid cartilage; />, diaphragma sellae; q, ravum sella;; r, sphenoidal sinus; 
s, middle meatus; /, rhinopharynx; u, Eustachian orifice; v, hard palate;_w, soft palate; 
x, uxula; y, anterior pillar of fauces; z, tonsillar fossa; aa, oropharynx; bb, epiglottis; cc, arye- 
piglottic fold; dd, larvngopharynx; ee, suprarimal portion of larynx; ff, ventricular band. 
gg, vocal band; /;/;, infrarimal portion of larynx; ii, I ricoid cartilage; ;'/- tracheal ring. 



upper and lower lips (labia), the former usually being in the form of a 
double curve, coming together at the median line and forming a small 
teat or tubercle, while the latter is made up of a single curve extending 
from angle to angle. While this general description applies to the 



THE BUCCAL ORIFICE. 3 

labial forms most frequently met with, this must not be taken for a con- 
stant condition. In some instances the lips are thin, with straight paral- 
lel margins, firmly set against the teeth, and seldom separated from 
each other when at rest. In another class they are thick, full, and promi- 
nent, with their margins strongly curved, resting lightly against the teeth, 
and more or less separated from each other during rest. Accompanying 
these extremes as well as the intervening conditions are various other 
peculiarities, such as the color, the rigidity or flexibility of the muscular 
structure, etc. The upper lip generally overhangs the lower, but in some 
instances the lower lip is the most prominent. Externally the lips are 
covered by the common integument, internally and over their contiguous 
surfaces by a continuation of the integument, the mucous membrane. 
Between the external and internal coverings and forming the substance 
of these fleshy folds are muscular fibers in which are imbedded numerous 
blood-vessels, nerves, and glands (labial glands). By the various muscles 
which enter into their construction the lips are loosely attached to the 
surfaces of the maxillary bones. 

The integument, or external covering of the lips, is similar to the 
skin covering other parts of the body. In the male it is subject to a 
peculiar change and modification of its outer layer, resulting in the pro- 
duction of a hairy growth. 

The mucous membrane, or internal covering of the lips, the begin- 
ning of which is strongly manifest by its bright-red color, is without moist- 
ure on the contiguous surfaces, is extremely sensitive, and contains a 
number of vascular papillae, many of which are accompanied by nerve 
terminals. Mucous membranes are described as lining certain cavities or 
tracts, as the digestive tract, the respiratory tract, and the genito-urinary 
tract, and it is upon the contiguous surfaces of the lips that the digestive 
tract begins. The line of junction between the integument and the mu- 
cous membrane is quite variable in form, but usually corresponds with the 
general curvature of the lips. Internally at the median line each lip is 
provided with a pronounced fold of mucous membrane, which is attached 
to the basal portion of the gum, the fir nu in of the lip (fraenum labium 
superioris and inferioris), which, in a measure, check the movements 
of the lips. 

Muscles of the Lips. 

The muscular fibers within the substance of the lips are principally 
those of a single muscle, the orbicularis oris, but associated with it is a 
portion of the elevator and depressor muscles of the lips, the levator 



4 ANATOMY. 

labii superioris alceque nasi, levator labii superioris, depressor labii inferi- 
oris or quadratus menti, and the zygomaticus minor. 

Orbicularis Oris. — This is the sphincter muscle which surrounds 
and controls the buccal aperture. In form it is an oval sheet with the 
long axis placed transversely, the fibers being continued from one lip to 
the other by passing around the angles of the mouth. It is divided into 
an internal or labial portion, and an external or facial portion. The 
labial portion forms the red part of the lips, and has no bony attachment 
except through the medium of the adjacent muscles. The external or 
facial portion forms the deeper layer and blends with the surrounding 
muscles, works in conjunction with them, and is provided with the follow- 
ing small bony attachments: The nasolabial slips are attached to the 
septum of the nose, other fibers are attached to the incisive fossa of the 
superior maxilla over the position of the lateral incisor tooth, and to the 
incisive fossa of the inferior maxilla near the socket of the lateral incisor 
or cuspid tooth. 

Structure. — The muscle consists of three sets of fibers, one of which 
runs transversely, one in a vertical, and one in an anteroposterior direc- 
tion. The transverse set is continuous with the fibers of the buccinator or 
cheek muscle, and forms the greater part of the muscle. The red or 
labial portion of the muscle is also formed from the same fibers, while the 
vertical fibers form the superficial part of the facial portion and are 
continuous with the fibers of the levator and depressor muscles. Some 
of these latter fibers pass around the corners of the mouth, thus becoming 
transverse, those from above passing to the lower lip, while those from 
below pass to the upper lip. The anteroposterior fibers pass from 
before backward between the transverse fibers, and unite the mucous 
membrane to the skin. These are chiefly found in the labial portion of 
the muscle. 

Relations. — The inner margin of the superficial surface is closely 
connected with the integument, while superimposed between this and the 
outer portion is a layer of fatty tissue. Upon the deep surface lies the 
mucous membrane of the mouth, separated from the muscular tissue by 
blood-vessels, the mucous glands, and small salivary glands. 

Action. — To bring the lips together, to draw the upper lip downward, 
and the lower lip upward; to draw together the corners of the mouth; 
to throw both lips outward; to draw them back against the teeth, and to 
oppose the action of all other muscles that blend into it and inclining to 
draw it in various directions. 

Levator Labii Superioris Alaeque Nasi. — As its name implies, this 



THE BUCCAL ORIFICE. 



muscle is an elevator of the upper lip and the wing of the nose. It is one 
of the superficial facial muscles, is thin and triangular, and is situated 
by the side of the nose, extending from the infra-orbital ridge to the 
upper lip. 



Frontalis 



Orbicularis oculi 



Procerus 
Quadr. labii sup. 
pars angularis 

Nasalis, pars transversa- 
Dilator naris anterior—; 
Dilator naris posteriori 
Quadr. labii sup. 
caput infraorbitale 

Caput zygomaticum 

Caninus -^hjl. 
Orbicularis oris 



Quadratua labii inferioris 
Triangularis 



Auricularis 
superior 



Auricularis 
anterior 




Fig. 2. — The Superficial Muscles of the Head and Neck. (Morris.) 



Origin. — From the nasal process of the superior maxilla near its 
orbital margin. 

Insertion. — From its origin it passes almost directly downward, 
dividing into two portions, the smaller of which is inserted into the nasal 
wing, while the larger portion is prolonged downward, blending into the 



6 ANATOMY. 

orbicularis oris and levator labii superioris, and forming a part of the 
substance of the upper lip. 

Relations. — Superficially, by the integument; deeply, by the levator 
anguli oris and compressor narium. 

Action. — By its smaller and shorter portion to raise the wing of the 
nose and to dilate the nostril; by its larger and longer portion to elevate 
the inner half of the upper lip. 

Levator Labii Superioris. — This muscle belongs to the superficial 
layer, and derives its name from its action. 

Origin. — From the facial surface of the superior maxilla, at a point 
between the orbital cavity and the infra-orbital foramen. Also by the 
attachment of a few fibers to the malar bone. 

Insertion. — Passing downward and inward, it is inserted into the orbi- 
cularis oris and the integument of the upper lip. Near its lower third 
it joins the levator labii superioris al&que nasi, and acts in conjunction 
with it. Occasionally it is reinforced by fibers from the orbicularis pal- 
pebrarum, which it receives at its outer border. 

Relations. — Superficially, by the orbicularis palpebrarum and the 
integument; deeply, by the levator anguli oris; the compressor nasi at 
its origin, and by the infra-orbital vessels and nerves. 

Action. — To elevate the upper lip. 

Levator Labii Inferioris. — This muscle, also known as levator 
menti, lies immediately beneath the mucous membrane of the lower 
lip, and can best be dissected by everting the lip and lifting off the 
membrane. 

Origin. — From the incisor fossa of the lower jaw at its upper border. 

Insertion. — Into the integument of the skin. 

Relations. — With the labial integument, the lower border of the orbi- 
cularis oris muscle, and its superficial surface with the oral mucous mem- 
brane. Deeply it is in close contact with the periosteum and the depres- 
sor labii inferioris. 

Action. — To raise and cause to protrude the integument of the chin. 

Depressor Labii Superioris. — This small muscle with its fellow of 
the opposite side is sometimes found within the mucous membrane form- 
ing the frenum of the upper lip. 

Origin. — It arises from the incisive fossa along its lower margin and 
some of its fibers are attached to that part of the alveolar process closely 
associated with the fossa. 

Insertion. — From the point of origin the fibers pass upward and are 
attached to the lower border of the nostrils and partition of the nose. 



THE BUCCAL ORIFICE. J 

Some of the fibers of this muscle are also attached to the integument cover- 
ing the wing of the nose. The balance pass downward and mingle with 
the fibers of the muscular structure of the upper lip. 

Relations. — At its point of origin the fibers are closely associated with 
the mucous membrane forming the gums. Above this the muscular 
structure of the lip overlies these fibers. Deeply it rests upon the surface 
of the superior maxilla, and joins its fellow of the opposite side at the 
median line. 

Action. — To depress the upper lip. 

Depressor Labii Inferioris, or Quadratus Menti. — The name 
of this muscle is derived from its form and action. It belongs to the 
second layer of facial muscles, is quadrilateral in shape, and consists of 
parallel fibers which meet above in the median line. 

Origin. — At the outer aspect of the lower border of the inferior 
maxilla, from a point near the symphysis to the space beneath the first 
bicuspid tooth. 

Insertion. — Its fibers pass upward and inward, and after uniting 
with its fellow of the opposite side, blend into the body of the orbicularis 
oris of the lower lip. 

Relations. — By its superficial surface with the integument and a 
portion of the depressor anguli oris; deeply, with the mental nerve and 
vessels, a portion of the orbicularis oris, the mucous membrane lining the 
lower lip, and the labial glands. 

Action. — To draw down and somewhat evert the lower lip. 

Zygomaticus Minor. — An extremely slender muscle belonging to 
the superficial set of facial muscles. It is closely associated with a larger 
muscle, the zygomaticus major, belonging to the angular series, to be 
described in connection with the muscles of the cheek. 

Origin. — From the anterior inferior part of one of the facial bones, 
— the malar, — close to its junction with the superior maxilla. 

Insertion. — It passes downward and forward, its fibers becoming 
lost in the special elevator muscle of the upper lip about midway between 
the median line and the angle of the mouth. 

Relations. — Superficially, by the integument, by its deep surface with 
the levator anguli oris, facial portion of the orbicularis oris, and the 
infra-orbital branch of the facial nerve. 

Action. — To elevate and somewhat evert the upper part of the lip. 
The Blood-supply to the Lips. 

The blood-supply to the lips is principally through the superior and 
inferior coronary arteries, both of which are branches of the facial artery. 



8 ANATOMY. 

In addition to these the inferior labial artery and the sub-mental, also 
branches of the facial, and the mental branch of the inferior dental artery 
supply a part of the lower lip. 

The superior coronary artery courses along the inferior margin 
of the upper lip, between the mucous membrane and the fibers of the or- 
bicularis oris muscle. At the median line it anastomoses with its fellow 
of the opposite side. 

The interior coronary artery, somewhat smaller than the superior, 
supplies the lower lip by coursing through its substance in a manner simi- 
lar to the superior coronary and also anastomoses with its fellow of the 
opposite side at the median line. 

Course of the Blood From the Heart to the Lips. — From the 
heart to the aorta, to the common carotid, to the external carotid, to the 
facial, to the superior and inferior coronary and the inferior labial arteries. 
After passing through the labial capillaries the blood is returned to the 
heart through the superior and inferior coronary veins, and the larger veins 
of which they are branches. 
Nerves of the Lips. 

The general nerve-supply to the lips is principally by small branches 
of the infra-orbital nerve for the upper lip, and by branches of the mental 
nerve for the lower lip. The buccal and superior maxillary branches of 
the lower division of the facial nerve supply the orbicularis oris muscle; 
the upper division of the facial nerve sends branches which supply the 
levator labii superioris alaeque nasi, as well as the levator labii superioris 
and the zygomaticus minor, while the superior maxillary branch of the 
lower division of the facial supplies the depressor labii inferioris. 



THE LATERAL WALLS OF THE MOUTH. 

The Cheeks {buccal). 

The cheeks are continuous with and similar in structure to the lips, 
being covered internally by mucous membrane and externally by the com- 
mon integument. Immediately beneath the mucous membrane are a 
number of transverse muscular fibers, covered externally by a layer 
of subcutaneous fat, and lying between this and the integument other 
muscular tissue, the fibers of which radiate in various directions, accord- 
ing to the action of the muscle to which they bslong. Besides muscular 
and fatty tissue, there are imbedded within the substance of the cheek 
blood-vessels, nerves, and glands. The fatty tissue spoken of as interven- 



THE LATERAL WALLS OF THE MOUTH. Q 

ing between the muscular fibers gives to the cheek the fullness and rotun- 
dity desired by so many but possessed by so few. 

The integument, or external covering of the cheek, is similar in 
structure to the skin covering other parts of the body, and, like the lips 
in the male, is productive of a hairy growth. 

The mucous membrane, or internal covering of the cheek, is similar 
to that of the lips, containing numerous glands (buccal glands) which are 
almost identical to, but smaller than, the labial glands. In addition to the 
buccal glands which are distributed over the entire membrane, there are 
about five of larger size, which open into the mouth in the region of the 
molar teeth, and are called molar glands (see Glands of the Mouth). 

The Muscles of the Cheeks (See Fig. 3). 

The transverse muscular fibers referred to as being immediately 
beneath the mucous membrane are those of the buccinator, a muscle 
named from its action, that of being the chief muscle employed by the 
trumpeter. External to the buccinator is the masseter, one of the muscles 
of mastication, the elevator and depressor muscles of the angle of the 
mouth, the levator anguli oris, and the depressor anguli oris, and the 
dermal muscles, zygomaticus major and zygomaticus minor, and the 
risorius. 

Buccinator. — This muscle forms the greater portion of the lateral 
wall of the mouth. It is deep-seated in the cheek, being one of the third 
stratum of facial muscles. 

Origin. — The fibers are distinct in their origin from a part of the 
alveolar process of the superior maxillary bone, at a point immediately 
over the second and third molar teeth, from the anterior border of the 
pterygomaxillary ligament, a narrow band of tendinous fibers or raphe 
extending from the pterygoid plate of the sphenoid bone to the mylohyoid 
ridge of the inferior maxilla near the position of the third molar tooth. 
Some of its fibers also arise from the outer wall of the alveolar process of 
the inferior maxilla below the second and third molars. 

Insertion. — The fibers pass forward and converge as they reach the 
lateral margins of the orbicularis oris; here the fibers of the upper portion 
pass downward and blend into the muscles of the lower lip, while the 
lower fibers pass upward and blend into those of the upper lip. Those 
fibers which arise from the inferior maxilla pass forward and also blend 
into the lower lip. 

Relations. — Superficially, by the skin and subcutaneous fat, the duct 
of Stenson, the masseter muscle, a portion of the angular group, and the 



IO 



ANATOMY. 



facial artery and vein. Passing over it are branches of the facial and buc- 
cal nerves, also a layer of deep fascia continuous with that which covers 
the upper part of the pharynx. By its deep surface it is in relation with 
the mucous membrane and buccal glands. 



Temporal 



Quadr.labii sup. 
caput angularis 



NasaliB, pars —* 
transversa i 

Caninus 

Depressor 

septi nasi 

Nasalis, pars 

alaris 

Orbicularis oris 



Zygomaticus 




Posterior belly 
of digastric 



Triangularis 

Quadratus la- 

bii inferioris 

Mentalis 

Mylo-hyoid 

Anterior belly 
of digastric 



Scalenus anterior 



FlG. 3.— The Deeper Muscles of the Face and Neck. (Morris.) 



Action.— To draw outward or backward the angles of the mouth, 
thus enlarging the buccal orifice and pressing the lips tightly against the 
teeth; to force the food between the occlusal surfaces of the molar and 
bicuspid teeth during mastication; to diminish the concavity of the cheek, 
compressing the air contained therein and forcing it forward. It be- 



THE LATERAL WALLS OF THE MOUTH. II 

comes an auxiliary in deglutition by shortening the cavity of the pharynx 
from before backward, through its connection with the superior 
constrictor. 

Masseter.— This muscle is placed immediately external to the 
buccinator, and is one of the principal muscles of mastication. It is 
short, thick, and somewhat quadrate in form, and is composed of two 
sets of fibers, superficial and deep. The fibers of the former are directed 
obliquely downward and backward; those of the latter, which are much 
shorter, pass almost vertically downward. 

Origin. — The superficial layer, from the malar process of the supe- 
rior maxilla, and from the anterior portion of the zygomatic arch of the 
malar bone. The deep layer from the posterior third of the zygomatic 
arch, as well as from the greater part of its inner surface. 

Insertion. — The superficial fibers are inserted into the ramus and 
angle of the inferior maxilla, and the deep fibers into the upper half 
of the outer surface of the ramus. 

Relations. By its external surface with the zygomaticus major, 
risorius, and platysma myoides muscles, the parotid gland and its duct; 
by the transverse facial artery, the facial vein, and facial nerve, and by 
the integument. By its internal surface with the ramus of the inferior 
maxilla, a mass of fat which separates it from the buccinator, and with 
the temporal muscle. Its posterior margin is in relation with the parotid 
gland, and its anterior with the facial artery and vein. 

Action. — The principal action of this muscle is to close the jaw and 
to draw it slightly forward. (For further description, see Muscles of 
Mastication, part i, chap. in. 

The Angular Series. — The remaining muscles of the cheek are 
those of the angular series, or those muscles which are inserted into the 
angle of the mouth, two coming obliquely from above, — the levator anguli 
oris and the zygomaticus major, — one running almost horizontally for- 
ward, — the risorius, — and one ascending from below, — the depressor 
anguli oris. 

Levator Anguli Oris. — This muscle, which receives its name from 
its action, belongs to the second layer of facial muscles. It is formed in 
the shape of a triangular sheet. 

Origin. — From the canine fossa of the superior maxilla, immediately 
below the infra-orbital foramen. 

Insertion. — Passing downward and outward it is inserted into the 
angle of the mouth, its fibers blending with those of the orbicularis oris 
and the other angular muscles. 



12 ANATOMY. 

Relations. — Superficially, with the levator labii superioris, the zygo- 
maticus minor, and the infra-orbital vessels and nerves; deeply, with the 
facial portion of the orbicularis oris and buccinator muscles, and the 
mucous membrane of the mouth. 

Action. — Especially to elevate the angle of the mouth, and to assist 
in drawing these angles inward, decreasing the size of the buccal orifice. 

Zygomaticus Major. — This muscle, the companion of which has 
been described in connection with the muscles of the lips, belongs to the 
first facial layer. It is composed of a long, fleshy band of muscular fibers, 
which run direct from their point of origin to their point of insertion. 

Origin. — From the malar bone, in close proximity to the zygomatic 
suture. 

Insertion. — From its origin it passes obliquely downward to the 
angle of the mouth, and blends into the fibers of the orbicularis oris and 
depressor anguli oris. 

Relations. — Superficially, with the skin and subcutaneous fat; deeply, 
with the malar bone, the masseter, and buccinator muscles, the facial 
and transverse facial arteries, the facial vein, and branches of the facial 
nerve. 

Action. — To draw upward and outward the angles of the mouth, 
as in smiling or laughing. By contracting, it throws into prominence the 
cheek tissues in front of the malar bone, and forces the lower eyelid up- 
ward. When acting simultaneously with its fellow of the opposite side, 
the buccal aperture is widened, and the upper lip is elevated, exposing 
the superior teeth. 

Risorius. — One of the superficial set of facial muscles, receiving its 
name from its supposed action in laughter (ridere, to laugh). It is flat 
and ribbon-shaped, and is frequently very small and poorly developed. 

Origin. — From the deep fascia covering the masseter muscle and 
parotid gland, some of its fibers occasionally arising from the mastoid 
process of the temporal bone. 

Insertion. — Passing transversely forward and inward to the angle of 
the mouth, its fibers blend with those of the orbicularis oris, and the 
depressor anguli oris. 

Relations. — Superficially, with the integument and subcutaneous 
fat; deeply, with the masseter and buccinator muscles, the facial artery 
and vein, and branches of the facial nerve. 

Action. — To draw the angles of the mouth directly outward, thereby 
increasing the width of the buccal orifice. 

Depressor Anguli Oris. — Also one of the superficial layer of facial 



THE LATERAL WALLS OF THE MOUTH. 13 

muscles, deriving its name in accordance with its action upon the angle 
of the mouth. It is a triangular-shaped muscle with its base below, be- 
coming narrow as it ascends. 

Origin. — From the lower border of the inferior maxilla, and from its 
external oblique line below the cuspid, bicuspid, and first molar teeth. 

Insertion. — Passing upward and inward it is inserted into the integu- 
ment at the angle of the mouth, its fibers blending into those of the muscles 
previously described as coming together at this point. 

Relations. — Externally, with the integument; deeply or internally, 
with the depressor labii inferioris, the buccinator, and the inferior coro- 
nary artery. 

Action. — To draw down the angle of the mouth and to slightly ex- 
tend it. 

Blood-supply to the Cheeks. 

The blood-supply to the cheeks is principally through the facial 
artery and its direct branches, the superior and inferior coronary, the 
transverse facial, and branches from the internal maxillary. 

The Facial Artery and Branches. — The facial artery, also called 
the external maxillary, enters the cheek after passing over the body of 
the inferior maxilla at the anterior edge of the masseter muscle. It 
courses obliquely forward and upward through the substance of the cheek, 
until it reaches the inner angle or canthus of the eye, where it joins the 
nasal branch of the ophthalmic artery, and is called the angular artery. 
Near the center of the cheek the inferior labial artery is given off, which 
passes forward and downward to the lower lip, but supplies a portion of 
the cheek in so doing. Midway between the center of the cheek and the 
angle of the mouth the superior and inferior coronary arteries are given off, 
supplying that part of the cheek immediately adjacent to the angle of the 
mouth, after which they pass on to supply the upper and lower lips. The 
masseteric branch is given off in the immediate center of the cheek, at a 
point immediately below the inferior labial, passes directly upward over 
the masseter muscle, and anastomoses with branches of the internal 
maxillary and transverse facial. There are also given off from the 
main trunk near the center of the cheek the buccal branches, which pass 
upward over the buccinator muscle, and also anastomose with branches 
of the internal maxillary and transverse facial arteries. 

The Transverse Facial Artery. — This is the largest branch of the 
temporal artery. It is at first deeply-seated in the substance of the pa- 
rotid gland, after leaving which it courses transversely over and supplies the 



*4 



ANATOMY. 



masseter muscle, sends off small branches which supply the integument 
of the cheek, and anastomoses with the buccal, infra-orbital, and the 



Orbicularis oculi muscl 



Transverse facial artery 

Zygomatieus minor 
muscle 

Zygomatieus major 
muscle 



Buccinator muscle 
Masseteric branch 

Masseter muscle 



Stylo-pharyngeus 
muscle 
Stylo-glossus muscle 

Ascending palat 
branch 

Tonsillar branch 

Extent at maxillary 
artery 
External carotid 
artery 
Posterior belly of 
digastric muscle 



Lingual artery 




Frontal branch of ophthalmic 
artery 

Nasal branch of aphtha, 

artery 



Angular artery 

Levator labii super- 
ioris alaeque nasi 
muscle 

Infraorbital artt ry 

Levator labii super- 
iors propriua 

La 1 1 ■ ral nasal or/cry 

Caninus muscle 



Artery of septum 
Superior labial 
artery 



Risorius muscle 



Iuii i tor labia/ artery 



Mi ntal branch of inferior 

ah < olar ai U ry 
Quadratus labii interioris 
muscle 
- Inferior labial artery 
Triangularis muscle 

Submental artery 
Branches to submaxillary 
aland 



Anterior belly of digastric muscle 

Mylo-hyoid muscle 

us muscle 



HYPOGLOSSAL XERVE 



Fig. 4. — Scheme of the Right External Maxillary Artery. (Morris after Walsham.) 



facial arteries. Besides the arteries already named, the deeper portions 
of the cheek receive blood from two branches of the internal maxillary 
artery, the masseteric branch and the buccal branch. The former supplies 



THE INTERIOR OF THE MOUTH. 1 5 

the masseter muscle and anastomoses with the masseteric branch of the 
facial, while the latter supplies the buccinator muscle and anastomoses 
with the buccal branches of the facial. 

Course of the Blood from the Heart to the Cheeks. — From the 
heart to the aorta, to the common carotid, to the external carotid, to 
the facial and its direct branches, or from the external carotid to the tem- 
poral, to the transverse facial and branches. 

From the cheeks the blood is returned to the heart principally 
through the facial vein, a division of the anterior superficial vein. It 
enters the cheek at a point midway between the lower eyelid and the wing 
of the nose, passes obliquely downward, being in close contact with the 
anterior edge of the masseter muscle over the body of the lower jaw, join- 
ing the internal jugular vein in the neck. The transverse facial vein 
which follows the course of the transverse facial artery, and the supe- 
rior and inferior coronary veins also collect and convey a portion of the 
blood from the cheeks to the larger veins and thence to the heart. 
Nerves of the Cheeks. 

The nerve-supply to the cheeks is principally from the seventh or 
facial nerve and its branches, the buccal branch supplying a greater part 
of their substance. There are also a few fibers of the infra-orbital branch 
of the seventh nerve distributed to the labiobuccal region. The buccal 
branch of the lower division of the facial, also the buccal branch of the 
inferior maxillary division of the fifth nerve, supplies the buccinator 
muscle. The infra-orbital branch of the upper division of the facial nerve 
supplies the zygomaticus major and the levator anguli oris; the buccal 
branch supplies the risorius, and the supramaxillary branch of the lower 
division of the facial nerve supplies the depressor anguli oris. 

THE INTERIOR OF THE MOUTH. 

For convenience of description the mouth may be divided into 
two parts — a superior portion and an inferior portion. In dissection this 
division may be accomplished by an incision beginning at the angles of 
the mouth and carried backward and slightly upward through the sub- 
stance of the cheeks until the temporomandibular articulation is reached. 
After disarticulating this joint, another incision is made, beginning at the 
joint on either side, carried downward and forward, then obliquely across 
the throat, until the two come together at the median line. This latter 
incision must be deep enough to completely sever the tissues of the throat. 

The superior portion of the mouth contains the hard palate, or roof 



1 6 ANATOMY. 

of the mouth, the soft palate, and the sixteen upper teeth, firmly set in the 
bone and surrounded by a dense fibrous tissue — the gums. The inferior 
portion contains the tongue and its attached muscles, forming the. floor 
of the mouth, the sixteen lower teeth and the gums surrounding them. 

THE SUPERIOR PORTION OF THE MOUTH (Fig. 5). 

The osseous framework, or base upon which this half of the mouth 
is constructed, is composed of a part of four bones — the two superior 
maxillary, or upper jaw bones, and the two palate bones (see Bones of 
the Mouth, p. 34). 

The Hard Palate, or Roof of the Mouth (Figs. 5 and 6). 

This is formed by the union of the palatal processes of the superior 
maxillary bones and the horizontal plates of the two palate bones at the 
median line. It is limited in front and laterally by the margins of the al- 
veolar process, or that portion of the bone which gives support to the 
teeth, and ends posteriorly in an irregular border, to which is attached a 
muscular, membrane-like curtain — the soft palate. The hard palate is 
covered throughout by a thick and firm mucous membrane, seldom so 
highly colored as that lining the lips and cheeks. The mucous mem- 
brane is closely adherent to the bone through its common covering, the 
periosteum. In the center of the hard palate is a ridge or fold of mucous 
membrane, which follows the median line from before backward; this 
is called the palatal or median raphe, and indicates the line of union 
formed during the development of the parts. Anteriorly, the raphe ends 
in a small papilla, which marks the opening of a canal in the bone — the 
anterior palatal canal. Posteriorly, the raphe usually diminishes, in size, 
but occasionally is well marked through the whole extent of the hard 
palate. Near the center of the hard palate it frequently separates into 
two or more smaller ridges, which are proportionately diminished in size, 
and are continued backward side by side. On either side of this central 
ridge, anteriorly, the mucous membrane presents a number of fantasti- 
cally arranged folds, the palatal ruga (wrinkles). These folds are 
usually quite numerous and prominent, but are occasionally but slightly 
developed. The nature of these wrinkles is strongly indicative of the char- 
acter or temperament of the individual; thus, in the four basal tempera- 
ments they may be divided as follows: in the bilious, heavy and 
strong, composed of angles rather than curves; in the nervous, few in 
number, close together, not prominent, and composed of long curves; 
in the sanguine, quite numerous, fairly prominent, well rounded and 



THE INTERIOR OF THE MOUTH. 



17 



graceful in outline; in the lymphatic, few in number, flat, widely sepa- 
rated, and but little curved. Acccompanying these varying conditions 
in the rugae will be found a corresponding variation in the raphe. The 
anterior and lateral margins of the mucous membrane covering the 
hard palate form the palatal portion of the gums, gingiva (the gum). 




Fig. 5. — The Superior Portion or Roof of the Mouth. 

In figure 6 the hard palate is shown with its mucoperiosteum re- 
moved. It will be observed that the bony plates are perforated by num- 
erous small openings or foramina, through which the body of the bone 
receives its nourishment, broken by depressions for the accommodation 
of the various mucous glands, and traversed by longitudinal grooves 
which give lodgment to blood-vessels and nerves. 

The arch formed by the hard palate from side to side varies greatly 



1 8 ANATOMY. 

in form, imparting much knowledge in regard to the temperament of 
the individual, and in a measure controlling the quality of the voice. 
Thus, in the sanguine temperament the roof of the mouth presents 
almost a perfect oval. In the bilious type it is comparatively high and 
flat, extending from the base of one alveolar process to another, from 
which point it descends abruptly to the necks of the teeth. In the ner- 
vous type the roof is high and semi-elliptical or parabolic in shape, and 
in the lymphatic it is low and segmental in form. 

In the same illustration the union or suture between the four bones 
may be seen at the median line. Near the anterior third of this central 
suture is the opening of the incisive or anterior palatal canal, the anterior 
palatal foramen, the location of which has been referred to in the descrip- 
tion of the mucous membrane. Near the posterior border, and situated 
within the suture which unites the superior maxillary bones with the palate 
bones (the palatomaxillary suture), are two other foramina, the posterior 
palatal; and immediately behind these, and separated by a thin ridge of 
bone, are the accessory palatal foramina, these being in the tuberosity of 
the palate bones. (For further description of these foramina, see Bones 
of the Mouth, p. 34.) By the vessels and nerves which enter the hard 
palate through these various foramina, the mucous membrane and 
glands receive their blood- and nerve-supply. 

Blood-supply to the Hard Palate. — This is principally derived 
from the posterior or descending palatal branch of the internal maxillary 
or deep facial artery, which passes downward in the posterior palatal 
canal and emerges through the posterior palatal foramen. Immedi- 
ately on reaching the palate it divides into an anterior and a posterior 
branch, the former passing forward in a groove provided for it to the an- 
terior palatal foramen, where it anastomoses with the nasopalatal artery. 
The groove in which the artery lies in its passage forward is usually at 
the base of the alveolar process, and in some instances is converted into 
a canal for a part of its length. The posterior branches pass backward 
and downward to supply the soft palate. In connection with supplying 
the hard palate proper, this artery carries blood to the palatal alveolar 
walls, to the mucous glands, the mucous membrane, and the gums. 

Course of the Blood from the Heart to the Hard Palate.— 
From the heart to the aorta, to the common carotid, to the external 
carotid, to the internal maxillary, to the posterior or descending palatal 
branch of the latter. From the hard palate the blood is returned to the 
heart by the superior palatal and inferior or descending palatal veins, the 
former following the course of the superior palatal artery, while the 



THE INTERIOR OF THE MOUTH. 19 

latter originates at a point near the junction of the hard and soft palates, 
passes downward, and joins the facial vein below the body of the in- 
ferior maxilla. 

Nerves of the Hard Palate, — The nerves of the hard palate are 
the anterior or large palatal and branches from the nasopalatal, both of 
which are branches of the sphenopalatal (Meckel's) ganglion. The an- 
terior palatal nerve arises from the inferior angle of the ganglion, passes 




Fig. 6. — The Hard Palate, or Roof of the Mouth, with its Membranous 

Covering Removed. 

downward, accompanied by the descending palatal artery, through the 
posterior palatal canal, from which it emerges at the posterior palatal 
foramen. From this point it passes forward in a groove of the hard 
palate, and joins the nasopalatal nerve as it emerges from the anterior 
palatal foramen. Accompanying this nerve in its course through the 
posterior palatal canal are other branches of the sphenopalatal ganglion, 
which pass to the soft palate, and will be described in that connection. 
The nerves of the hard palate are all sensory, and filaments are dis- 
tributed to the mucous membrane and glands and to the palatal portion 
of the gums. 

THE SOFT PALATE (Figs. 6 and 7). 

The soft palate is attached to the posterior border of the hard palate, 
from which it is continued as a backward prolongation. Hanging down- 



20 ANATOMY. 

ward, with its free borders inclining backward, it may be considered as 
forming a part of the posterior boundary of the mouth. It partially 
separates the mouth from the nasal cavity and from the pharynx. It is 
attached laterally to the walls of the pharynx, while its lower border is 
free. The substance of the soft palate is composed of a number of thin, 
but dense, muscular fibers, blood-vessels, nerves, and mucous glands, the 
latter being similar to those of the hard palate. The anterior surface 
of this muscular curtain is concave, directed forward and downward, 
and is traversed by a median raphe. The posterior surface is convex, 
directed backward and upward, and is continuous with the nasal cavity. 
Suspended from the center of its free border is a small rounded or conic 
membranous appendix, the uvula, and passing outward from the base 
of this at each side are two curved muscular folds which extend outward 
and downward, and are known as the pillars of the fauces. From the 
position which these folds occupy they are divided into the anterior and 
the posterior pillars of the fauces. The anterior pillar is formed from 
muscular fibers which extend from the soft palate to the side and base of 
the tongue (palatoglossus muscle), and is somewhat prominent as it passes 
downward, outward, and forward. The posterior pillar approaches 
more closely to its fellow of the opposite side than does the anterior. 
It is formed of muscular fibers which extend from the soft palate above 
to the pharynx below (palatopharyngeus muscle). It is somewhat con- 
cave in its downward and backward course, and while closely united to 
the anterior pillar above, is separated from it below, leaving a triangular 
interval or niche in which is lodged a small, almond-shaped body, the 
tonsil, the space being known as the tonsillar recess. The intervening 
space — bounded by the margins of the soft palate above, the root of the 
tongue below, and the pillars laterally — is called the isthmus of the fauces, 
which establishes the communication between the mouth and the pharynx. 
The free margins of the soft palate, assisted by the pillars of the fauces, 
mark the posterior boundary of the mouth. The entire surface of the soft 
palate and its prolongations, the pillars of the fauces, is covered with 
mucous membrane, being continuous with that of the mouth on its an- 
terior surface, and with that of the nasal cavity on its posterior surface. 
Muscles of the Soft Palate. 

On each side the muscles of the soft palate are the palatoglossus, 
palatopharyngeus, levator palati, and tensor palati, together with the 
azygos uvulce. 

Palatoglossus. — A small fasciculus of fibers, somewhat cylindric 
in form, expanding at either end into a thin sheet. It is named from its 



THE INTERIOR OF THE MOUTH. 



21 



attachment to the soft palate and to the tongue. It is the prominence of 
this muscle, together with its covering of mucous membrane, that forms 
the anterior pillar of the fauces. 



M. uvulae 
Hamular process 

Fharyngo- 
palatiuus 




AUDITORY OR 

EUSTACHIAN TUBE 



Levator veli 
palatini 



Pharyngo-palatinus 



Constrictor pharyngis 
superior 



Crico-arytaenoideus 
posterior 



Thyreoid cartilage 



Cricoid cartilage 



TRACHEA 



ESOPHAGUS 



Fig. 7.— View of Muscles of Soft Palate, as seen from within the Pharynx. 
{Morris Modified from Bourgery.) 

Origin. — By a thin muscular sheet from the under surface of the 
aponeurosis of the soft palate near the median line, its fibers uniting 



2 2 ANATOMY. 

with those of the opposite side. It passes downward in front of the ton- 
sil and against the pharyngeal wall. 

Insertion. — Into the side and base of the tongue. 

Relations. — Superficially, it is covered by the mucous membrane 
of the soft palate and tongue; deeply, in contact with the aponeurosis 
of the soft palate, the superior constrictor muscle of the pharynx, and 
one of the muscles of the tongue — the hyoglossus. 

Action. — The lateral walls of the soft palate are drawn down, and 
the sides of the tongue are drawn upward and slightly backward. Acting 
in conjunction with the palatopharyngeus, the opening of the fauces is 
constricted. 

Palatopharyngeus. — This muscle — also named from its attach- 
ments — is broad above, where it forms the greater part of the lower 
half of the soft palate. Near the median line a few of its fibers blend 
with those of its fellow of the opposite side. 

Origin. — By two heads from a point near the raphe or median 
line of the soft palate, passing downward and slightly backward, 
forming, with its covering of mucous membrane, the posterior pillar of 
the fauces. 

Insertion. — Into the posterior border of the thyroid cartilage, and to 
the inner surface of the lower part of the pharynx. 

Relations. — In the soft palate, superficially, with the mucous mem- 
brane, both anteriorly and posteriorly; above, with the levator palati; 
and beneath, by the mucous glands. In the posterior pillar it is sur- 
rounded with mucous membrane, and in the pharynx by the constrictor 
muscles of the pharynx and the mucous membrane. 

Action. — To constrict the opening of the fauces, by bringing together 
the posterior pillars, thus depressing the soft palate and elevating the 
pharynx. It controls the position of the soft palate during respiration, 
and elevates the pharynx during deglutition. 

Levator Palati. — This is a moderately thick muscle, and derives 
its name from its action upon the soft palate. 

Origin. — By a short tendon from the under surface of the petrous 
portion of the temporal bone, and from the posterior ahd inferior aspect 
of the cartilage of the Eustachian tube. 

Insertion. — After passing downward by the side of the posterior nares 
it is inserted into the median line of the soft palate, where its fibers 
unite with those of its fellow of the opposite side. 

Relations. — Externally, with the tensor palati and superior con- 
strictor muscles; internally and posteriorly, with the mucous membrane. 



THE INTERIOR OF THE MOUTH. 23 

Action. — To raise the soft palate, bringing it against the posterior 
wall of the pharynx. 

Tensor Palati. — This is a slender and flattened muscular sheet, 
and receives its name from its action upon the soft palate. 

Origin. — From the scaphoid fossa at the base of the internal ptery- 
goid plate and the spinous process of the sphenoid bone; also from the 
outer side of the anterior aspect of the Eustachian tube. 

Insertion. — After descending between the internal pterygoid muscle 
and the internal pterygoid plate, and winding around the hamular process 
of the latter, it is inserted into the transverse ridge on the horizontal 
portion of the palate bone, and at the median line of the soft palate, 
where it is continuous with the aponeurosis of the opposite side. 

Action. — To tighten or spread the soft palate laterally, forming a 
septum between the posterior nares and the pharynx. It also opens the 
Eustachian tube during deglutition. 

The azygos uvulae (so named because it was at one time supposed 
to be a single muscle) is composed of a pair of small muscles which orig- 
inate from the aponeurosis of the soft palate and the nasal spine of the 
palate bone. They pass downward and form or are inserted into the 
uvula. 

Relations. — Anteriorly, with the levator palati, palatoglossi, and a 
part of the palatopharyngei; posteriorly, to the mucous membrane. 

Action. — To shorten or draw up the uvula. 
Blood-supply to the Soft Palate. 

The posterior or descending palatal branch of the deep facial artery. 
after emerging from the posterior palatal canal, sends its posterior divi- 
sion backward and downward to the soft palate, in the substance of which 
they anastomose with the ascending palatal artery. After passing over 
the superior border of the pharynx, the ascending pharyngeal artery sends 
off branches which are distributed to the soft palate. A few branches of 
the superior palatal branch of the internal maxillary and a few twigs 
from the lingual artery also convey blood to the parts. 

Course of the Blood from Heart to the Soft Palate. — From the 
heart to the aorta, to the common carotid, to the external carotid, to the 
facial or lingual, to the various branches named above, to the soft palate. 
The return of the blood to the heart is principally through the superior 
and inferior or descending palatal veins, both of which closely follow the 
course of the arteries of the same name. 

Nerves of the Soft Palate. — The small, or posterior, palatal, the 
external palatal (both of which are branches of Meckel's ganglion). 



24 



ANATOMY. 



branches of the glossopharyngeal nerve, and the following nerves which 
supply the various muscles: filaments from the pharyngeal plexus to the 
palatoglossus and palatopharyngeus, branches of the Vidian to the leva- 
tor palati and azygos uvula?, and from the mandibular division of the fifth 
nerve to the tensor palati.* 

THE INFERIOR PORTION OR FLOOR OF 
THE MOUTH (Fig. 8). 
This half of the cavity of the mouth contains the tongue and its 
attached muscles, the sixteen lower teeth firmly implanted in the jaw, and 




Fig. 8. — The Inferior Portion or Floor of the Mouth. 

the gums covering the alveolar walls. The base or osseous framework 
upon which this portion of the mouth is constructed is principally made 
up of a single bone, the inferior maxillary, mandible, or lower jaw bone 

* A description of the upper teeth will be found in another chapter 



THE INTERIOR OF THE MOUTH. 25 

(see Bones of the Mouth, p. 34). The hyoid bone, situated between the 
angles of the mandible in the upper part of the neck, and at the base of 
the tongue, giving attachment to many of the muscles about the floor of 
the mouth, may also be considered in this connection. The floor of the 
mouth is bounded anteriorly by the lower lip, laterally by the cheeks, 
and below by the muscles attached to the external and internal oblique 
lines of the mandible. 



THE TONGUE (Fig. 9). 
The tongue is a freely movable, highly sensitive, muscular organ. It 

True Vocal Cord Posterior Wall of the Pharynx Corniculum Laryngis 



False Vocal 
Cord. 



Tonsil 



Adenoid Tissue 
at Base of Tongue 



Foramen Caecum 



Cuneiform 
Cartilage 



Epiglottis 




Median Glosso- 
epiglottidean Fold 



Fungiform Papillae Circumvallate Papillae 
Fig. 9. — Superior Aperture of Larynx. (Deaver.) 

assists in the function of speech, participating, also, in the special sense 



26 



ANATOMY. 



of taste, and in mastication and deglutition. The organ is attached 
posteriorly to a U-shaped bone, the hyoid bone, which of itself is movable, 
and is placed in the neck between the angles of the lower jaw and the 
thyroid cartilage. 

The tongue is suspended and kept in its position in the mouth by 
numerous muscles, some of which are attached to the base of the skull, 
and others to the lower jaw and hyoid bone. 

The size of the tongue bears little or no relation to the size of the indi- 
vidual, but is proportioned to the capacity of the alveolar arch, which space 

it completely fills when at rest. The 
shape of the tongue is controlled by 
the shape of the alveolar arch; thus, 
when the arch is contracted, narrow, 
and pointed, the margins of the 
tongue, when at rest, will assume 
that form (a, Fig. 10); but when the 
arch is broad and rounded anteriorly, 
the margins of the tongue will also 
be broad and rounded (b, Fig. 10). 

The substance of the tongue is 
chiefly composed of muscular fibers, 
which are arranged in a complicated 
manner, crossing one another at various angles, thus making the 
movements of the organ exceedingly varied and extensive. Fibrous, 
areolar, and fatty and glandular tissue enter into its structure, and it is 
freely supplied with blood-vessels and nerves. Its free surface is covered 
by a specialized mucous membrane, and over its entire surface art- 
numerous mucous follicles and glands. 

Before continuing the description of the tongue, it will be necessary 
to name its parts. The upper surface, or that facing the roof of the 
mouth, is the dorsum; those portions directed toward the cheeks are 
known as the sides of the tongue; while the hem which unites these two 
borders at the median line, and extending a short distance backward 
on either side, is the tip. That portion between the frenum and extend- 
ing back to the pillars of the fauces is the base, while that part of the dor- 
sum immediately posterior to the tip is the post-lip, that region which 
lies between the post-tip and the base being the prebase. The dorsum, 
sides, and tip are free, while the base is attached by muscles to the lower 
jaw and hyoid bone. 

From the base to the epiglottis is a fold which serves to limit the 




Fig. io. 



THE INTERIOR OF THE MOUTH. 27 

movement of the latter organ, and to the sides of the base the pillars of 
the fauces are attached. Under the anterior free extremity in the median 
line is a fibrous muscular lamina or ligament, the frenum (frenum linguae), 
which connects this part of the organ with the lower jaw and marks the 
anterior border of the base of the tongue. The tongue is divided through 
its anterior two-thirds by a slight longitudinal furrow, the median raphe, 
which ends posteriorly near a small foramen, not constant in the adult 
tongue, but plainly observed in the fetus or infant, the foramen ccecuni. 
This foramen represents the upper termination of the thyreoglossus duct. 

Papillse of the Tongue. — Over the anterior two-thirds of the dor- 
sum and the sides and tip of the tongue are a number of small, soft, 
conic eminences, wh ch are known as the papillae of the tongue. These 
are most numerous over the anterior part of the dorsum, and at the back 
they are covered and partly hidden by an epithelial coating. In general, 
the papilla? are quite similar to those of the integument, not being com- 
pound organs in their vascular and nervous supply. In consequence of 
their variation in form, and arrangement the papillae are variously 
named. The largest papillae, being arranged like the letter V, are called 
the circumvallate or jalciform; those of medium size, the fungiform, are 
so named from their resemblance to a young mushroom; and the smallest 
and most numerous are known as the conic or filiform papillae. Each 
papilla presents a broad, free end, and is attached by a constricted base, 
which rests in a small, cup-like concavity, about the margins of which is a 
well-formed circular rim. Beneath the thick epithelium of these parts 
are numerous secondary papillae, and about the base of each papilla 
are the openings of one or more glands. 

The circumvallate or jalciform papillae (Fig. 9) form a V-shaped line 
at the posterior boundary of the dorsum. They are few in number 
(varying from six to twelve), but are largest in size, not infrequently meas- 
uring 1/4 of an inch in diameter. These papillae are generally regarded 
as being gustatory, or directly interested in the sense of taste. Each 
papilla is capped with a small secondary papilla. 

The fungiform papillae (Fig. 9), of medium size, varying from 1/20 
to 1/50 of an inch in diameter, are scattered over the dorsum, sides, and 
tip of the tongue at irregular intervals, and are much more highly colored 
than the smaller papilla? which surround them. They vary greatly in 
number, and being principally gustatory, account in a great measure for 
the diversity in the acuteness of the sense of taste in different individuals. 
These papilla 1 , like the circumvallate, are capped with smaller secondary 
papilki?. 



28 ANATOMY. 

The conic ox filiform papillae (Fig. 9) are the smallest and most numer- 
ous, and are thickly scattered over the entire surface of the dorsum in 
front of the circumvallate as well as over the sides and tip of the tongue. 
They are placed closely together, and with such regularity that they 
fairly ridge the tongue with delicate lines, which run parallel with the 
circumvallate in that region, but as the tip is approached they become 
transversely inclined. These papillae are generally regarded as being 
tactile or directly interested in the sense of touch, and are concerned in 
directing the movements of the food during mastication. They also pos- 
sess secondary papillae upon their surfaces. 

Immediately posterior to the circumvallate papillae are two shallow 
grooves which follow the V-shaped line of the papillae and unite at the 
foramen caecum. These grooves serve to indicate the line of junction 
between the anterior and posterior portions of the tongue. The latter not 
being within the cavity of the mouth will not be described. 

Muscles of the Tongue (Fig. 11). 

The muscles of the tongue are both extrinsic-outward or external, and 
intrinsic-inherent, inward, or special. 

The extrinsic muscles include those which have their origin from 
the base of the skull, the hyoid bone, or the mandible, and are the hyo- 
glossus, geniohyoglossns, styloglossus, palatoglossus, and a few fibers of 
the superior constrictor of the pharynx. The intrinsic muscles which 
make up the bulk of the tongue are two in number, the superior lingualis 
and inferior lingualis. 

Hyoglossus. — As its name implies, this muscle extends from the 
hyoid bone to the tongue. Its fibers are so arranged that they form a 
thin square sheet. 

Origin. — It arises from the whole length of the upper border of the 
great cornu, from the body, and by a few fibers from the lesser cornu of 
the hyoid bone. At their point of origin the fibers are in the form of a 
thin sheet, and ascend toward the tongue almost parallel to one another, 
but before reaching the tongue the anterior fibers pass slightly forward, 
and at the upper margin of the side of the tongue bend inward and join 
the fibers of the superior lingualis. In their distribution to this part of 
the tongue they form a kind of submucous covering to the organ. 

Insertion. — Into the posterior half of the side of the tongue, between 
the styloglossus and superior lingualis muscles. 

Relations. — Externally, with the digastricus, styloglossus, stylohyoid, 
and mylohyoid muscles, the lingual and hypoglossal nerves, Wharton's 



THE INTERIOR OF THE MOUTH. 



2 9 



duct, and the sublingual gland. Internally, with the lingualis, geniohyo- 
glossus, and middle constrictor of the pharynx muscles, the lingual artery, 
and the glossopharyngeal nerve. 

Action. — To extend the tongue and to draw it backward, also to 



Stylo -glossus 



DORSUM OF TONGUE 



Genio-hyo-glossus 
Genio-hyoid 




Cut edge of mylo-hyoid 



EPIGLOTTIS 
(indicated by dotted tines) 



GREATER CORNU OF HYOID BONE 



STYLOID PROCESS 

Stylo-hyoid 

POSTERIOR PORTION 
OF TONGUE 



Tjjr - Stylo-pharyngeus 



Hyo-glossus 

Thyro-hyoid 
ligament 

CARTILAGO TRITICEA 



Thyro-hyoid 
membrane 



THYROID CARTILAGE 



Median portion of 
crico-thyroid 
membrane 

CRICOID CARTILAGE 
FIRST RING OF TRACHEA 



rn ww r ff n 

Fig. 11. — Side View of the Tongue, with its Muscles. (Morris.) 



draw downward the sides of the tongue, making its dorsum more convex 
transversely. 

Geniohyoglossus (Fig. 11). — This muscle also receives its name 
from its three points of attachment, the chin internally, the hyoid bone, 
and the tongue. It is a triangular-shaped muscle, narrow and pointed 
at its attachment to the mandible, and broad and fan-shaped on approach- 
ing the tongue. Being near the median line, it is separated from its 



3° 



ANATOMY. 



fellow of the opposite side by a thin layer of connective tissue, the septum 
of the tongue. 

Origin. — It arises by a short tendon from the upper genial tubercle 
of the lower jaw, from which point its fleshy fibers diverge fan-like to its 
extensive insertion. 

Insertion. — To the whole length of the tongue from base to apex 

Vertical and Trans- 
Mucous Submucous Superior Lingualis ve . rse Musc " lar Fibers 

Membrane Tissue Muscle 




Inferior Lingualis Muscle 



Ranine Arteries Vena Comes 
Intrinsic Muscular Fibers 

Fig. 12.— Transverse Section of One-half of Tongue. {Denver.) 

immediately external to the median line, into the body of the hyoid bone, 
and by a few fibers into the side of the pharynx. 

Relations. — By its inner surface, with the septum of the tongue and 
its fellow of the opposite side; by its outer surface, with the hyoglossus, 
mylohyoides, styloglossus, and lingualis muscles, sublingual gland, lin- 
gual artery, and hypoglossal nerve. Superiorly, with the mucous mem- 
brane of the floor of the mouth; inferiorly, with the geniohyoid muscle. 



THE INTERIOR OF THE MOUTH. 



3 1 



Action. — Its anterior fibers assist in drawing back the tip of the 
tongue, its posterior fibers throwing forward and protruding the tongue. 
This muscle also depresses the center of the dorsum longitudinally, 
making it concave transversely, and some of the lower fibers which are 
attached to the hyoid bone elevate the bone and assist in raising the tongue. 

Styloglossus. — Also named from its attachment, is a long fan-shaped 
muscle, somewhat compressed laterally. 



FR/ENUM LING'Jyfc 



Linguahs inferior 



Hyo-gloseus 



Genio-hyoid rj/ 



Mylo-hyoid, reflected 



Stemo-hyoid 




- LingualiB inferior 



Genk-hyo-glossus 



Stylo-glosBus 

Hyo-glossus 
BODY OF HY010 BONE 
Genio-hyoid 



THYROID CARTILAGE 



Fig. 13. — Under Surface of the Tongue with Muscles. {Morris.) 



Origin. — From its point of origin at the tip of the styloid process of 
the temporal bone, and from a portion of the stylomaxillary ligament, it 
passes with a long curve, forward, slightly downward, then upward and 
inward to its place of insertion at the side of the tongue. 

Insertion. — Upon reaching the side of the tongue it divides into two 
portions, the fibers of one portion passing transversely inward, while the 
others pass longitudinally along the side of the tongue. 

Relations. — Externally, with the internal pterygoid muscle, parotid 



32 ANATOMY. 

and sublingual glands, lineal nerve, and the mucous membrane of the 
mouth; internally, with the superior constrictor and hyoglossus muscles, 
and with the tonsil. 

Action. — To draw the tongue backward and to produce a transverse 
concavity to its upper surface by elevating its sides. 

Superior Lingualis. — This is one of the intrinsic muscles, and is 
situated immediately beneath the mucous membrane, extending from the 
base to the tip of the organ. 

Inferior Lingualis (Fig. 13). — This muscle is placed near the under 
surface of the tongue, and is composed of two bands which extend from 
base to apex, some of its fibers being attached posteriorly to the hyoid 
bone, and in passing forward are placed between the hyoglossus and genio- 
hyoglossus. Anteriorly, its fibers blend with those of the styloglossus. 

Many of the fibers of this muscle run transversely and are placed 
between the two former intrinsic muscles. These, together with some 
fatty tissue compose the greater part of the substance of the tongue. 
The fibers are attached at the median line to the fibrocartilaginous sep- 
tum of the tongue, and laterally to the mucous membrane. In connection 
with the transverse fibers there are a few placed vertically, which pass by 
long curves from the dorsum to the under surface of the tongue. 

Blood-vessels of the Tongue (Fig. 14). 

This organ receives its blood principally through the lingual, facial. 
and ascending pharyngeal arteries. The lingual artery arises from the 
front of the external carotid near the facial, and often as a common trunk 
with it. From its point of origin to the tongue it is divided into three 
portions, the first or oblique, the second or horizontal, and the third or 
ascending, and it is this latter portion which directly supplies the tongue. 
Ascending tortuously beneath the hyoglossus muscle, it reaches the under 
surface of the tongue, and, lying between the lingualis and hyoglossus 
muscles, it is continued to the under surface of the tip of the tongue, at 
which point it is called the ranine artery. At a point about corresponding 
with the posterior margin of the hyoglossus muscle a branch is given off 
(the dorsalis lingua), which passes almost directly upward, and, after 
dividing into two or more small branches, supplies the back part of the 
dorsum of the tongue and the mucous membrane about the circumvallate 
papillae. At the anterior border of the hyoglossus muscle another branch 
is given off (the sublingual artery) supplying the anterior muscular struc- 
ture of the floor of the mouth. The facial artery by one of its muscular 
branches supplies the styloglossus muscle. 



THE INTERIOR OF THE MOUTH. 



33 



Course of the Blood From the Heart to the Tongue. — From the 
heart to the aorta, to the common carotid, to the external carotid, to the 
lingual artery, and its smaller branches to the tongue. From the tongue 
the blood is returned to the heart principally through the lingual vein, 
which begins at the ranine vein beneath the tip of the tongue, passes back- 
ward under cover of the mucous membrane, following the course of the 
lingual artery until the hyoglossus muscle is reached, beyond which point 



Posterior br. of 
descending palatine A 

Palatine br. of 
a,s cen dm <j pharyngeal A- -Al/ 

Ascending pharyngea 

A seen ding pa la I in e br 
of facial A 

Tonsillar br. 

of facial A. 

Stylo pharyngeus M- 

Facial A. 

Middle conslriclorM- 
Dorsalis linauae A. 

Lingual A- 
£x tern a I caro lid A . 



-Descending palatine .4. 

-Anterior br. of descending palatine A. 
z Stylo-glussusM. 

,1'alato -glossusM. 

, Tonsillar br. dorsalis linguae A . 




Superior thyroid 

Infra - hyo id br. ofsfyp. thyroid A ^ffL 



astricM. 

oftingualA. ^lyiv-nyoid M 

Fig. 14. — Arteries of Tongue and Tonsil 



ry of fraenuni 
Submental A. 
Gewo-hyoid M. 
Genio -hyo-glossus M 
Sublingual A . 

(Deaver.) 



the fibers of the muscle separate the artery from the vein. After receiving 
the sublingual and dorsalis linguae veins, the course of which corresponds 
to the arteries of the same name, the vein passes backward and down- 
ward and empties into the internal jugular. 

Nerves of the Tongue. 

The mandibular division of the fifth nerve by its lingual branch sup- 
plies the papillae of the anterior portion and sides of the tongue, while the 
lingual branch of the glossopharyngeal supplies the circumvallate papillae, 
the base, and posterior sides. A few branches of the superior laryngeal 
are distributed to the back part of the root of the organ. The motor 
nerve of the tongue is the hypoglossal or ninth, supplying both the extrinsic 
and intrinsic muscles.* 

* A description of the lower teeth will be found in another chapter. 



CHAPTER II. 

The Bones of the Mouth: The Superior Maxillae, The Palate Bones, 
The Inferior Maxilla or Mandible. 

SUPERIOR MAXILLARY BONES. 

The superior maxillary bones, two in number, one on each side of 
the median line or center of the face, are irregular in shape, and may be 
classed as the largest bones of the face, with the probable exception of 
the mandible or inferior maxilla. From the central position which they 
occupy they contribute largely to the bony framework of this portion of 
the skull. They are not only instrumental in forming the major portion 
of the roof of the mouth or hard palate, but assist in the formation of 
the floor of the orbit, and the sides and base of the nasal chamber. They 
furnish a solid and firm foundation for the sixteen upper teeth, and by 
their variety in form contribute much to the character and quality of the 
voice. The outer or facial surface of these bones provides attachment 
for numerous muscles. Each superior maxillary bone presents for 
examination a body, four surfaces, and four processes. The body may be 
described as forming an irregular triangle, its general contour depending 
much upon the temperament of the subject and consequent character 
of the teeth. Within the body of the bone is an irregular cavity, the maxil- 
lary sinus or antrum of Highmore. 

The four surfaces of the body of the bone are the superior or orbital, 
the lateral or facial, the proximal or nasal, and the posterior or zygomatic. 

The Superior or Orbital Surface, which assists in forming the 
greater portion of the floor of the orbit, is slightly concave over its anterior 
two-thirds, and somewhat convex over the remaining or posterior third. 
The three borders of this surface form almost an equilateral triangle, and 
are named, as indicated by their location, the anterior, the posterior, and 
the mesial or proximal. The anterior border is convex from before back- 
ward and slightly concave throughout its length. That portion which 
forms a part of the lower border of the completed orbit is smooth, while 
the remaining portion is roughened to form an articulation with the malar 
bone. The posterior border extends from the center of the malar process 
backward and inward to the orbital process of the palate bone, which 

34 



SUPERIOR MAXILLARY BONES. 35 

articulates with the superior maxillary at this point. A portion of this 
border, together with the orbital process of the palate bone, is instrumen- 
tal in forming the anterior boundary of the sphenomaxillary fissure. 

The mesial or proximal border is marked by an irregular thin edge, 
which articulates with a portion of two bones, the lacrimal anteriorly, 
and the os planum of the ethmoid bone posteriorly. Only the posterior 
two-thirds of this border presents an articulating edge, the remaining or 
anterior third being smooth and forming the commencement of the lacri- 
mal groove, which in the articulated skull becomes a canal, passing 
downward and backward to communicate with the inferior meatus of 
the nose. Beginning at the posterior border of this surface and running 
forward will be found a deep groove — the infra-orbital groove. When 
near the center of the surface, this groove dips down and is covered by a 
layer of bone, from which point it passes forward as a canal — the infra- 
orbital canal — making its exit at a point about \ of an inch below 
the border of the orbit, near the center of the facial surface of the 
bone, the foramen thus formed being the infra-orbital foramen. Near 
the root of the nasal process, and immediately within the anterior border 
of this surface, is a small depression which marks the origin of the in- 
ferior oblique muscle of the eyeball. 

The Lateral or Facial Surface (Fig. 15). — This surface is made 
up of the anterior part of the bone; it is irregularly concave, and pre- 
sents a greater variety in form than any other part of the bone, with the 
single exception of the palatal process. It is bounded above by the infra- 
orbital ridge, and the roughened surface of the malar process which ar- 
ticulates with the malar bone; below, by the border of the alveolar process; 
anteriorly, by the frail concave border of the opening into the nasal cavity, 
the anterior nasal spine, and the perpendicular margins of the bone be- 
neath. Posteriorly, this surface is separated from the posterior or zygo- 
matic surface by a strong projecting eminence, the malar process. 

The canine fossa is a deep depression, situated almost in the center 
of this surface, the bone at this point being extremely thin and closely re- 
lated to the floor of the antrum. The concave floor of this fossa is fre- 
quently traversed by one or two smaller convex ridges, corresponding to 
the roots of the bicuspid teeth. 

The canine eminence is a prominent ridge running vertical to the 
body of the bone immediately anterior to the canine fossa, and corre- 
sponding in position to the root of the cuspid tooth, the size and type of 
the tooth having much to do with its extent and prominence. This 
ridge gives origin to one of the depressor muscles of the upper lip, and also 



36 



ANATOMY. 



to one of the depressor muscles of the wing of the nose. The incisive or 
myrtiform fossa is a depression found between the canine eminence and 
the inner margin of the bone. The depth of this fossa is in a measure 
controlled by the position and size of the teeth, and by the amount of 
prominence in the canine eminence. j"* 

The infra-orbital foramen, which transmits the infra-orbital nerves 
and blood-vessels, is immediately below the center of the infra-orbital 



Nasal Process 



Infra-orbital 
Foramen 



Canine Fossa 



Nasal Spine 
Facial Suiface 

Incisive Fossa 

Canine 
Eminence 




Orbital Surface 



Zygomatic 
Surface 

Malar Process 

Posterior Dental 
Canals 



Tuberosity 



Fig. 15. — Left Superior Maxilla, Outer or Facial Surface. 



ridge, and near the upper margin of the canine fossa. It is oval in form, 
and faces almost directly toward the median line. Between this foramen 
and the infra-orbital ridge is the point of origin for the principal levator 
muscle of the upper lip, the levator labii superioris proprius. The whole 
extent of the facial surface may present a number of vertical ridges, or the 
same space may be regular and smooth, the condition being controlled 
by the size and shape of the tooth-roots and the thickness of the bone 
covering them. One of the levator muscles of the angle of the mouth, 
the levator anguli oris, is attached to this surface near the upper border 
of the canine fossa. 



SUPERIOR MAXILLARY BONES. 



37 



The Proximal or Nasal Surface (Fig. 16). — Above, this surface 
presents a large, irregular opening into the maxillary sinus, this opening 
being nearly closed in the articulated skull by neighboring bones. In 
front of the opening into the sinus, and standing perpendicular from the 
body of the bone, is the strong ascending plate of the nasal process, 
marked near its lower extremity by a rough, horizontal ridge, the inferior 



Antrum of 
Highmore 



Inferior Meatus 



Posterior Pala 
tine Groovt 

Palate Proces> 




Nasal Process 



Ridge for Mid- 
dle Tuibinal 

Middle Meatus 

Thin plate of 
bone over Lac- 
rymal Groove 

Ridge for Infe- 
rior Turbinal 



Nasal Spine 



Anterior Pala- 
tine Groove 



Fig. 16. — Left Superior Maxilla, Internal, Proximal, or Nasal Surface. 



turbinated crest, which gives attachment to the inferior turbinated bone. 
The smooth, concave surface immediately above this ridge corresponds 
to the middle meatus of the nose, and forms the external wall of that pas- 
sage. Below the opening into the sinus and the nasal process, and occupy- 
ing the anterior two-thirds of the middle of this surface, is a large semi- 
circular space, forming the outer wall of the inferior meatus of the nose. 
Below this space, and projecting inward from the body of the bone, is 
the palatal process, which articulates with the corresponding process 



38 ANATOMY. 

of the opposite bone. At the anterior superior angle of the nasal surface, 
and passing downward just behind the nasal process, is the lacrimal 
groove. In the articulated skull this groove becomes a canal, the lacri- 
mal canal, the ethmoid and the inferior turbinated bones assisting in its 
formation. The canal passes downward and slightly backward, and 
opens into the inferior meatus of the nose. It is about 1/2 of an inch in 
length, and gives passage to the lacrimonasal duct. 

The lacrimal tubercle is a small prominence of bone formed at the 
junction of the anterior border of this surface, with the external surface 
of the nasal spine. The extended portion of the lacrimal duct, the 
lacrimal sac, finds lodgment at this point. 

The posterior palatine or palatomaxillary canal commences near the 
middle of the posterior border of this surface, appearing in the disartic- 
ulated bone as a groove, and, passing downward and forward, gives pas- 
sage to the posterior palatine vessels and anterior palatine nerves. The 
canal is made complete by the articulation of the superior maxillary with 
the vertical plate of the palate bone. On the posterior portion of the nasal 
surface, extending from the irregular opening into the antrum downward 
to a point opposite the palatal process, is a roughened surface about 1/2 
of an inch in width, which marks the extent of articulation with the 
palate bone. 

The proximal or nasal surface presents four borders — superior, in- 
ferior, anterior, and posterior. The superior border is irregular, and 
articulates with the lacrimal and ethmoid bones. The inferior border 
projects inward, and forms a strong horizontal plate — the palatal process. 
This process defines the border from before backward to the posterior 
third, at which point it is marked by the lower border of the roughened 
surface which articulates with the palate bone. The anterior border is 
sharp, frail, and irregular in outline, and forms the free margin of the 
opening into the nasal cavity. The posterior border is marked by the 
inner margin of the zygomatic surface, being smooth upon its upper half, 
and roughened upon its lower half, at which point it articulates with the 
palate bone. 

The Posterior or Zygomatic Surface (Fig. 16). — This surface is 
partly convex and partly concave, and is bounded above by a well-de- 
fined margin, which serves as the dividing-line between this and the 
superior or orbital surface. This border is also marked by a roughened 
margin on the posterior portion of the malar process, the orbital portion 
of the palate bone articulating at this point. The major portion of this 
border is smooth and rounded, forming the lower border of the spheno- 



SUPERIOR MAXILLARY BONES. 39 

maxillary fissure, and marked by a notch, the commencement of the infra- 
orbital groove. The outer border of the surface is formed by the malar 
process, and by a line drawn from this point directly downward to the 
alveolar process. The inner border is smooth and somewhat irregular 
above, while below it is roughened for articulation with the palate bone. 

The tuberosity, which also forms a portion of the inferior border of 
this surface, is a roughened and rounded eminence of bone, and is pene- 
trated by a number of nutrient vessels, which enter the many small fora- 
mina at this point. Between the tuberosity and the body of the zygomatic 
surface are several large apertures leading into canals, which pass into, 
and give nourishment to, the substance of the bone. These canals trans- 
mit the posterior dental blood-vessels and nerves, one of which, after 
passing over the outer wall of the maxillary sinus, unites with the anterior 
dental canal. The tuberosity is posterior to, and above, the third molar 
tooth, in some instances extending directly backward from this tooth for 
the distance of half an inch or more, but usually the tooth penetrates the 
base of the tuberosity, leaving but a thin layer of bone posterior to it. 

The inferior border of the posterior or zygomatic surface is formed 
by that portion of the alveolar process which supports the second and 
third molar teeth. 

The bone presents four processes for examination — the nasal, the 
malar, the palatal, and the alveolar. 

The nasal process is a strong, irregular piece of bone, standing ver- 
tically above the body of the bone proper, and forming the lateral boundary 
of the nose. This process is greatly increased in strength by the infra- 
orbital ridge joining it at or near its base, and ascending its external 
anterior surface to some extent. That portion of the process posterior 
to its junction with the infra-orbital ridge assists in forming the inner 
wall of the orbit. 

The external or anterior surface of the nasal process is marked by a 
number of shallow grooves, traces of the development of the bone. 
Scattered over this surface are a number of small foramina, the entrances 
to minute canals transmitting nutrient vessels to the body of the bone. 
This surface gives origin to one of the lip muscles, the levator labii superi- 
oris alaeque nasi. 

The internal surface of the nasal process is usually described as in- 
cluding all that portion between the superior border and the floor of the 
anterior nares. The surface is marked by two concave portions and 
two ridges. The two ridges divide the surface into three parts — the 
superior meatus, the middle meatus, and the inferior meatus of the nose. 



40 ANATOMY. 

The superior meatus is the smallest of the three, and occupies the slightly 
concave space above the superior ridge. The middle meatus, partly 
concave, and partly convex, includes the space between the superior 
and the inferior ridges, and extends from the free margin of the bone in 
front to the lacrimal groove behind. The inferior meatus, which is 
much the largest, occupies all that concave surface between the inferior 
ridge above and the palatal process below, and extends from the anterior 
margin of the bone backward to the point of articulation with the palate 
bone. The two ridges previously referred to are known as the superior 
turbinated crest, which articulates with the middle turbinated bone, and 
the inferior turbinated crest, which articulates with the inferior turbinated 
bone. 

The malar process is a large, irregular portion of bone situated 
at the angle of separation between the facial and zygomatic surfaces, 
and presents a triangular, roughened surface for articulation with the 
malar bone. The superior boundary of this process is formed by the 
orbital surface and the outer end of the infra-orbital ridge; the inferior 
boundary may be marked by an irregular imaginary line running from 
the upper margin of the canine fossa to a point between the first and second 
molar teeth, while the posterior inferior boundary may be traced from 
the outer superior angle of the zygomatic surface downward and forward 
to the point above referred to. This process, as well as the nasal process, 
is subject to much variety in form and general outline. The malar 
process, assisting as it does in forming what is commonly called the cheek 
bone, is particularly variable in size, and in certain types and races it is 
so prominent as to become a controlling feature in the facial form. One 
of the muscles of mastication — the masseter — has a portion of its origin 
from the malar process. 

The palatal process is more directly interested in the formation 
of the cavity of the mouth than any other portion of the superior maxillary 
bone. By articulating with its fellow of the opposite side, it forms about 
three-fourths of the hard palate or roof of the mouth, the remaining fourth 
being formed by a portion of the palate bones. It is thick and strong, and 
projects horizontally inward from the inner surface of the body of the 
bone. It presents two surfaces for examination — a superior or nasal 
surface, and an inferior or oral surface. The superior or nasal surface 
is smooth and more or less concave, and forms the floor of the nares. 
The inferior or oral surface is also concave, but is much roughened by 
numerous small projections, between which are lodged the mucous 
glands. Upon the anterior portion of this surface are a number of 



SUPERIOR MAXILLARY BONES. 4 1 

small foramina, which mark the entrance to numerous small canals 
giving passage to nutrient vessels to supply the body of the bone. Near 
the center of the posterior third are the anteroposterior grooves, which 
accommodate the posterior palatine nerves and blood-vessels. This 
process also presents for examination three borders and various other 
points of interest. The three borders are the interior,' posterior, and 
mesial. The anterior border is thick and somewhat irregular; the posterior 
border is thin and frail, and articulates with a portion of the palate 
bone. The mesial border presents a wide articulating surface in front, 
behind it is narrow, the whole extent of this border articulating with the 
corresponding process of the opposite bone. 

The Nasal Spine. — At the anterior superior angle of the palatal 
process is a well-defined spine — the nasal spine — being formed by a pro- 
longation of the process beyond the level of the facial surface of the bone. 
This process, when articulated with its fellow of the oposite side, forms 
the base of the nose. 

The Nasal Crest. — Beginning at the base of the nasal spine, and ex- 
tending backward along the median border of the bone, is a sharp, irre- 
gular ledge of bone, the nasal crest. This portion of the process ar- 
ticulates with the vomer. 

The incisor crest is a continuation of the nasal crest anteriorly, pro- 
jecting beyond the nasal spine in the form of a sharp, spear-like point. 

The incisive foramen, ox foramen of S tens on, is situated immediately 
back of the incisor crest, and leads downward and forward from the nasal 
chamber toward the mouth, entering that cavity just back of the central 
incisor tooth. This passage in the single bone is a simple groove, but 
in the articulated skull it becomes the anterior palatine canal, which, 
after passing downward, opens on the nasal surface of the palatal process 
by four foramina — the incisive foramina, and the foramina of Scarpa, or 
the naso-palatine foramina. These foramina transmit the naso-palatine 
nerves. 

The palatal process of the superior maxilla is subject to a greater 
variety in form than any other portion of the bone, this variation in the 
articulated skull being the cause of the many different curves assumed 
by the roof or dome of the mouth. 

The Alveolar Process. — This process forms the lower margin of 
the bone, and extends from the base of the tuberosity behind to the 
median line in front, at which point it articulates with the same process 
of the opposite bone. It has an outer and an inner margin corresponding 
to the buccal and palatal surfaces of the roots of the teeth, which are 



42 ANATOMY. 

firmly imbedded in it. Its general form from before backward is that of 
a gradual curve, somewhat variable in different bones, the extent of this 
variation depending on the type or race to which the bone belongs. The 
body of the process is made up of an outer and an inner plate, which are 
connected by numerous septa of cancellated bone. The outer plate of 
the process is continuous with the facial and zygomatic surface of the body 
of the bone, and assists in forming these surfaces. It is quite thin and 
frail, and the position of the alveoli beneath are well shown by the numer- 
ous vertical ridges upon it. The inferior margin of the outer plate is 
reinforced by an additional thickness of bone, forming the border of the 
alveolar sockets. That portion of the plate supporting the molar teeth 
is heavier in general, and the position of the sockets are not so plainly 
outlined. The inner plate of the alveolar process is much heavier and 
stronger than the outer plate, and extends from the margins of the alveoli 
below to the palatal process above. The inferior margin of this plate is 
not reinforced except in the region of the molars. The construction of 
the inner plate is, to a great degree, controlled by the shape and position 
of the palatal process. In the lymphatic temperament this process, 
when articulated with its fellow, forms a flat or shallow dome to the oral 
cavity, and in so doing gradually curves into the alveolar process, giving 
it additional thickness. The depth of the process in this type is not 
great, and the roots of the teeth are short and heavy in proportion. In 
the bilious temperament the inner alveolar plate is deep and abrupt, ex- 
tending from the inferior margin upward in almost a perpendicular direc- 
tion to the palatal process which joins it almost at right angles. The 
alveolar process gives origin to one of the cheek muscles — the buccinator 
— which is attached to the outer plate near its upper margin, and directly 
over the space occupied by the second bicuspid and first molar teeth. 
The Alveoli or Tooth Sockets. — These cavities, which are variable 
in number, are formed by the outer and inner plate of the alveolar process, 
and by numerous connecting septa of bone placed between the two plates. 
The shape and depth of each cavity is regulated by the form and length 
of the roots of the teeth which they support. The first socket, or that 
next to the median line, gives support to the central incisor tooth. It 
forms almost a perfect cone, and has an average depth of half an inch. 
Its lower border is circular, and the anterior or labial portion describes 
a larger circle than the posterior or palatal half. The mesial and distal 
walls are somewhat flattened. The second cavity, proceeding backward 
from the median line, supports the lateral incisor tooth. It is also conic, 
but somewhat smaller than the preceding. It is seldom over 3/8 to 5/16 



SUPERIOR MAXILLARY BONES. 43 

of an inch in depth. It is much flattened on its mesial and distal walls, 
giving the appearance of an oblong, rather than a round, cavity in trans- 
verse section. This socket, as well as that for the central incisor, occupies 
an almost vertical position in the process. Very frequently the socket 
for the lateral incisor presents a slight distal curve at its upper extremity. 
The third socket, or that giving support to the cuspid tooth, is much 
larger and deeper than either of those previously described. It extends 
upward, inward, and backward with an average depth of 5/8 to 3/4 of 
an inch. In transverse section, its labial wall presents a much larger 
circle than its palatal margin. The labial and distal walls are much 
flattened and somewhat convex. The general direction of this socket is 
slightly to the distal. The socket which supports the first bicuspid is 
usually divided from mesial to distal by a thin septum of bone, thus 
forming an outer or buccal socket, and an inner or palatal socket. This 
division seldom exists to the full depth of the cavity, but usually begins 
about midway of its length. The lower margin of this socket is oblong or 
egg-shaped, its outer or buccal portion forming a larger curve than its 
palatal. The lateral walls are slightly concave or flattened, until the 
point of separation is reached, when they become more circular, the alveoli 
above this point becoming cone-shaped. It is not uncommon for this 
socket to be a single cavity, and when thus formed it resembles a flattened 
cone, with the buccal and palatal margins rounded. The next socket 
gives support to the second bicuspid tooth, in most instances being a 
single cavity, but in rare instances it is divided near its upper extremity. 
In general outline it resembles the socket for the first bicuspid. 

The socket for the first molar is much larger than any of those 
previously described; its inferior margin presents a circular outline on its 
buccal and palatal portions, the former curve being larger than the latter. 
The mesial and distal walls are flattened and slightly concave. The 
upper three-fourths of this socket is divided into three separate compart- 
ments, being so arranged that two are upon the buccal and one upon the 
palatal side. The septa separating the two buccal cavities from the pala- 
tal cavity are heavy and strong, while that placed between the two buccal 
sockets is thin and frail. The two buccal cavities are usually flattened 
upon their mesial and distal sides. The palatal socket is larger and 
somewhat deeper than the buccal, the average depth of all being about 
1 1 2 of an inch. The socket for the second molar is similar in most respects 
to that for the first molar, except that it is somewhat smaller. The same 
description might answer for the third molar socket, which in general 
is similar to the alveoli for the other molars. It is smaller than the second 



44 ANATOMY. 

molar socket, and may be a single cavity, or it may be divided into three 
compartments. 

Articulations. — The superior maxillary bone articulates with 
its fellow of the opposite side, with the frontal, lacrimal, ethmoid, palate, 
vomer, malar, and inferior turbinated bones. Occasionally it articulates 
with the sphenoid bone. 

Attachment of Muscles. — The muscles attached to this bone are 
eleven in number, and are as follows: 

Compressor nares, Internal pterygoid, 

Orbicularis oris, Orbicularis palpebrarum, 

Levator labii superioris akeque Levator labii superioris proprius, 

nasi, Inferior oblique, 

Levator anguli oris, Buccinator,. 

Depressor ate nasi, Masseter. 

Blood-supply. — The maxilla receives its vascular supply from 
numerous large arteries. They are derived from the alveolar, infra- 
orbital, nasopalatal, descending palatal, ethmoidal, nasal, frontal, and 
facial branches. 

Development. — The superior maxilla arises from four points of 
ossification, which are deposited in membrane. These four centers 
make their appearance as early as the eighth fetal week, this early begin- 
ning making it somewhat difficult to accurately follow its growth. The 
four centers are named, as located, premaxillary, maxillary, malar, and 
prepalatal. The premaxillary nucleus gives rise to the incisive portion 
of the bone, or that part supporting the incisor teeth. During early life 
this division of the bone is separated from the body of the bone, and is 
known as the premaxillary portion (Fig. 17). Union between the pre- 
maxillary portion and the maxilla proper takes place about birth, and the 
suture thus formed is visible on the facial surface until the sixth or seventh 
year, and on the palatine surface until the adult period. The palatal 
suture extends as far back as the posterior border of the anterior palatal 
canal. This nucleus also sends a narrow process upward which forms 
part of the outer boundary of the anterior narial aperture. On the 
palatal aspect it furnishes a speculum which surrounds the anterior and 
mesial walls of Stenson's canal. The posterior limit of the premaxillary 
portion is indicated by the suture on the palatal surface. The maxillary 
nucleus forms the greater portion of the body of the true maxilla and the 
nasal process. The malar center gives origin to the malar process, 
and all that portion external to the infra-orbital groove. The prepalatine 



3RARY 

SUPERIOR MAXILLARY BONES. 4S 

W.v.u, 

center gives rise to the nasal surface of the bone and that portion of the 
palatal process posterior to Stenson's canal. 

Development of the Alveolar Process. — This process is represented 
at birth by the walls of a deep groove, in which are lodged the partly 
calcified, deciduous teeth and the germs of most of the permanent teeth 
(see Development of the Teeth). 

The growth of the process continues with the growth of the teeth 
until, finally, at about the seventh month after birth, the dental organs 



Premaxillan 
Portioi 

Suture 



Palatal Portion 




Malar Portion 



Fig. 17. — Left Superior Maxillary, about the Third Year, Enlarged. 

are completely encased within its walls. With the decalcification of the 
roots of the deciduous teeth comes the loss of the process surrounding 
them, and, as the permanent teeth advance to take their place in the 
arch, the process is again built up about their roots. 

The Maxillary Sinus, or Antrum of Highmore* (Fig. 16). — This 
is a large cavity situated within the body of the maxilla. Its general 
shape is that of a pyramid, with its base directed toward the median line, 
or nasal surface, its apex pointing toward and extending into the malar 
process, and, in some instances, penetrating the malar bone. The size 
of the cavity varies in different subjects and in the opposite bone of 

* Described separately, in preference to including in general description of the bone. 



46 



ANATOMY. 



the same subject. The average capacity is about three fluidrams, but 
this may be increased to six or eight fluidrams. The size of the bone and 
the prominence of the malar process control, in a measure, the size of 
the cavity; but not infrequently the largest bone will present the smallest 
sinus. Sex also appears to exert a controlling influence over the capacity 
of the cavity, it being greater in the male than in the female. In youth 
the cavity is quite small, the walls being much thicker proportionately 
than in the adult. The walls of the sinus in the matured subject are 
quite thin and frail, and are four in number. The superior wall is formed 




FlG. 18. — Developing Maxillary Bones about the Fifth Month after Birth. 



by a thin plate of bone, the floor of the orbit. This surface is almost flat, 
and serves as a roof to the cavity. Near the anterior margin of this sur- 
face is a thick rib of bone which marks the course, and forms one of the 
walls of the infra-orbital canal. 

The inner wall, or that looking toward the nasal surface, is formed 
by the thin bony layer separating this cavity from that of the nares. The 
outer or lateral surface, formed by the facial and zygomatic surfaces of 
the bone, is smooth, and convex from before backward. Near the center 
of this surface the cavity may penetrate the malar process, and in the dis- 
articulated skull would present an opening at this point. The inferior 
wall is formed by the alveolar process, and is marked by a number of 
irregular eminences corresponding to the roots of the neighboring teeth. 
The teeth referred to are generally the first and second molars, and occa- 



THE PALATE BONE. 47 

sionally the second bicuspid. It is not unusual for the roots of one or more 
of these teeth to penetrate the floor of the sinus, in consequence of which the 
lining membrane of the cavity may suffer disease generated in the teeth. 

The inferior wall is much the strongest of the four, and, besides the 
unevenness of the surface produced by the tooth-roots, it frequently 
supports a number of thin, bony partitions, which may completely or 
partly divide the floor of the cavity into numerous small compartments. 

The posterior portion of the lateral wall is marked by the posterior 
dental canals, which give passage to the posterior nerves and blood-vessels. 
In like manner the anterior portion of the lateral wall is grooved for the 
reception of the anterior dental nerves and blood-vessels. Upon the inner 
wall, or that forming the base of the pyramid, is an opening which commu- 
nicates with the middle meatus of the nose. In the articulated skull this 
opening is quite small, being from 1/8 to 1/4 of an inch in diameter. 
The correct idea of this opening can not be obtained by studying the 
individual bone, as the numerous perforations then to be observed are 
closed or partly closed by articulation with adjacent bones. The mucous 
membrane lining the nasal cavity enters the sinus through the small 
aperture above referred to, and forms a continuous lining over its entire 
surface. Cryer has thrown much light upon the relations of the maxillary 
sinus to the mouth and teeth, and he has demonstrated beyond a doubt 
that the relationship existing between the parts is susceptible to extensive 
variation. He has shown that in some instances the cavity upon one 
side will be large, with its floor broken by the tooth-roots, while that upon 
the opposite side will be extremely small and far removed from the root 
apices. In fact, these researches have so revolutionized .the subject 
under consideration that the foregoing description is only reliable in so 
far as it treats of the conditions most frequently met with. 

THE PALATE BONE. 

The palate bones (Fig. 19), two in number, are situated immediately 
posterior to the two maxillae, and with them complete the hard palate. 
They also assist in forming the boundaries of the orbital and nasal cavities, 
the sphenomaxillary, the sphenopalatine, and the pterygoid fossa, the 
sphenomaxillary fissure, the posterior ethmoidal cells, and the maxillary 
sinus. When in position in the skull, these bones are wedged between 
the maxillae and the sphenoid bone. They are rectangular in outline, and 
each bone presents for examination a horizontal and a vertical plate, a 
tuberosity, and two processes, the orbital and the sphenoid. 



48 



ANATOMY. 



The horizontal plate, smaller than the vertical, assists in forming 
the hard palate, and corresponds to the palatal process of the maxilla. 
In entering into the construction of the hard palate the form of this plate 
varies to the same degree as the palatal plate of the maxilla. In general, 
it is described as quadrilateral in shape, having two surfaces and four 
borders. The superior surface, which is concave from side to side, forms 
the posterior floor of the nasal chamber. The inferior surface completes 
the hard palate posteriorly, and presents, near its posterior border, a 
transverse ridge for the attachment of one of the muscles of the soft 
palate (the tensor palati) ; the anterior border is serrated for articulation 




*CRjOSITV 
PTERYGOID FOSSA 

Fig. 19. — The Two Palate Bones in their Natural Position, 
Dorsal View. (Testut.) 

with the palatal process of the maxilla. The posterior border is free, 
curved, and sharp, and marks the posterior boundary of the hard palate. 
At the median line this border terminates in a sharp point, which, when 
articulated with the corresponding bone of the opposite side, forms the 
posterior nasal spine; to this point the azygos uvula* muscle is attached. 
The external border is situated just below the junction of the horizontal 
and vertical plates. In this portion is a groove which assists in forming 
a portion of the posterior palatal canal. The internal border is broad 
and serrated for articulation with its fellow of the opposite side. When 
the palate bones are in position in the skull, these borders form a ridge, con- 
tinuing the crest formed by the palatal process of the maxilla, this crest 
receiving the inferior border of the vomer. 

The vertical plate is thin and frail and extends from the floor of 
the nasal chamber below to the upper extremity of the sphenopalatine 
notch above. It has two surfaces and four borders. 



THE PALATE BONE. 49 

The external surface is roughened for articulation with the maxilla, 
excepting a small triangular surface near the upper extremity, which 
forms a portion of the sphenomaxillary fossa, and a small portion near 
the middle of the surface close to the anterior border, which forms a 
portion of the wall of the maxillary sinus. Near the posterior boundary 
of this surface is a vertical groove, which forms, when articulated with the 
maxilla, the posterior palatal canal, transmitting the descending palatal 
nerves and vessels. 

The internal surface is divided into three shallow depressions by two 
transverse ridges — the superior and inferior turbinated crests. The 
lower depression thus formed assists in the construction of a portion of 
the interior meatus of the nose. The crest immediately above this depres- 
sion articulates with the inferior turbinated bone. The central depres- 
sion, the largest of the three, forms a portion of the middle meatus of the 
nose, the crest above articulating with the middle turbinated bone. The 
superior depression — much smaller but deeper than either of those pre- 
viously described — forms a large part of the superior meatus. The an- 
terior border of the vertical plate is thin and sharp, the inferior turbinated 
crest protruding near the center of the border, and forming the maxillary 
process. This process assists in closing the maxillary sinus by being 
received into the maxillary fissure of the maxilla. At the upper extrem- 
ity of this border is the orbital process, which presents for examination 
five surfaces, three of which are articular. The anterior or maxillary 
surface is directed outward, upward, and downward. It is oblong in 
form and articulates with the posterior superior angle of the inner surface 
of the maxilla. The posterior or sphenoidal surface is directed back- 
ward, upward, and inward, and articulates with the vertical plate of the 
ethmoid bone. The superior or orbital surface is triangular in form, 
extending upward and outward, forming the posterior angle of the floor 
of the orbit. The external or zygomatic surface is smooth, oblong, and 
directed outward, backward, and downward, forming a portion of the 
sphenomaxillary fossa. 

The posterior border of the vertical plate is irregular and serrated, 
and comes into relation with the internal pterygoid process, terminating 
below in a prominent tuberosity. This presents three grooves or flutes. 
The inner receives the internal pterygoid, the outer the external pterygoid 
process, while the middle groove completes the pterygoid fossa, and 
gives attachment to a portion of the internal pterygoid muscle. This 
process also gives rise to the superior constrictor of the pharynx. 
Passing through the tuberosity are a number of small canals, those on the 



50 ANATOMY. 

nasal side being the accessory palatal canals. Near the junction of the 
tuberosity with the horizontal plate is the opening of the posterior palatal 
canal, and beyond this the small external palatal canals. 

The sphenoidal process is at the superior end of the posterior border. 
It is variable in shape and curves upward, backward, and inward. It 
presents a superior, an external and an internal surface, and two borders 
— an anterior and a posterior. 

The superior surface, the smallest of the three, is marked by a groove, 
which assists in forming the sphenopalatine canal. This surface articu- 
lates with the horizontal portion of the sphenoidal turbinated bone. 
The external surface assists in forming the sphenomaxillary fossa by its 
anterior portion, while the posterior portion is rough for articulation 
with the pterygoid plate of the ethmoid bone. The internal surface is in- 
strumental in forming a portion of the outer wall of the posterior nares, 
and for this purpose is smooth and concave. The anterior border 
forms the posterior margin of the sphenopalatine notch. The posterior 
border is serrated, and articulates with the inner surface of the pterygoid 
process. 

The superior border of the vertical plate is divided by a deep notch or 
foramen, which divides the orbital from the sphenoidal process. This 
opening is the sphenopalatine notch or foramen, and transmits the spheno- 
palatine vessels and nerves from the sphenopalatine fossa to the nasal 
chamber. 

The inferior border of the vertical plate joins the external border of 
the horizontal plate. Extending downward and backward from the 
inferior and posterior borders is the pyramidal process, the borders 
of which are serrated for articulation with both pterygoid plates of the 
sphenoid bone. 

Articulations. — The palate bone articulates with the sphenoid, 
superior maxilla, sphenoidal turbinated, inferior turbinated, ethmoid, 
and with its fellow of the opposite side. 

Attachment of Muscles. — The following muscles are attached to 
the palate bone: 

Tensor palati, Internal pterygoid, 

Azygos uvulae, Superior constrictor of pharynx. 

Blood-supply. — The arteries which supply this bone are derived 
from branches of the descending palatine, the sphenopalatine, and 
pterygopalatine. 

Development. — The palate bone is developed from a single center 



INFERIOR MAXILLARY BONE. 



51 



deposited in membrane. This center makes its appearance about the 
eighth or ninth fetal week, near the line of junction between the horizon- 
tal and vertical plates. At birth these plates are about the same length, 
but soon after this period, when the nasal sinuses increase in height, 
the vertical plate begins to lengthen, and continues to do so until it be- 
comes nearly double the length of the horizontal plate. 

INFERIOR MAXILLARY BONE. 

The Inferior Maxillary, Mandible, or Lower Jaw Bone (Fig. 20). — 
This bone, having no osseous union with the skull proper, may be con- 




Fig. 20. — The Mandible or Inferior Maxilla. Right Side, External or Facial Surface. 
a, Sigmoid Notch; b, Coronoid Process; c, Condyle; d, Neck of Condyle; e, Ramus; 
/, External Oblique Line; g, Angle; h, Mental Foramen; i, Mental Protuberance; j, Body. 

sidered as one of its appendicular elements. It is the heaviest and 
strongest bone of the head, gives support to the sixteen lower teeth, and 
serves as a framework for the lower half or floor of the mouth. It is 
situated at the lower extremity of the face, and immediately below the 
superior maxillary and malar bones, while its posterior extremity rests 



52 ANATOMY. 

against the glenoid fossa of the temporal bone, forming a movable articu- 
lation with this cavity. In general, the bone is symmetric in outline, 
and presents for examination a horizontal portion, or body, and two 
vertical portions, or rami, which in the adult are almost perpendicular to, 
or at right angles with, the body of the bone. 

The body, or horizontal portion, consists of two identical halves, 
which meet at the median line and form a slight vertical ridge, the sym- 
physis. This line indicates the point of union between the two lateral 
halves, which at birth are usually separated, but soon after this period 
become firmly united. Each lateral half of the body presents two sur- 
faces — an external and an internal; and two borders — a superior and an 
inferior. 

The external or facial surface (Fig. 20) is smooth and convex, 
and furnishes a number of points for examination. Beginning at the 
median line, the symphysis ends inferiorly in a prominent triangular 
surface — the mental protuberance, or chin. 

The Incisive Fossa. — Passing backward from the symphysis, and 
immediately above the triangular ridge which forms the mental process, 
is a decided but shallow depression — the incisive fossa. This fossa 
gives origin to one of the elevator muscles of the chin — the levator menti. 
Slightly posterior to and below this fossa, on a line corresponding to the 
position of the cuspid tooth, is an oblong depression for the origin of the 
depressor muscle of the lower lip — depressor labii inferioris. 

The External Oblique Line. — Extending obliquely across the facial 
surface from the mental process to the base of the vertical portion of the 
bone, and continuous with its anterior margin, is a well-defined ridge— 
the external oblique line. Near the center of this ridge, or below the 
position occupied by the bicuspid and first molar teeth, is the point of 
attachment of the depressor muscle of the angle of the mouth — the de- 
pressor anguli oris. Somewhat anterior to and above this point is the 
origin of the depressor muscle of the lower lip — the depressor labii inferi- 
oris. Between the line of origin of the depressor anguli oris and the in- 
ferior border of the bone is a roughened surface for the attachment of 
the platysma myoides muscle. This roughened surface divides the body 
of the bone into an upper and a lower portion. That portion above is 
known as the alveolar or mucous portion, while that below is called the 
basilar or non-mucous portion. The attachment of the platysma myoides 
muscle at this point marks the lower boundary or floor of the mouth. The 
superior or alveolar portion of the bone is within the cavity of the mouth, 
and is covered with mucous membrane and mucoperiosteum; while the 



INFERIOR MAXILLARY BONE. 53 

inferior or basilar portion is outside and below the cavity, and is covered 
with periosteum similar to other bones. 

The Mental, or Anterior Dental Foramen. — Midway between the 
superior and inferior border of the body, and usually below the second 
bicuspid tooth, is a large foramen — the mental or anterior dental fora- 
men — giving passage to the mental branches of the inferior dental nerve 
and accompanying blood-vessels. The position of this foramen is not 
constant, but, as previously stated, it is usually below the second bicuspid, 
or between this point and the first bicuspid. The buccinator muscle, 
which forms a large portion of the lateral wall of the mouth, has its origin 
from the facial surface of the mandible, being attached to the alveolar 
portion immediately below the molar teeth. 

The Internal Surface of the Body of the Bone (Fig. 21). — The 
median line is marked by a slight vertical depression, representing the line 
of union, and corresponding to the symphysis externally. 

The Mylohyoid, or Internal Oblique Ridge. — The internal surface is 
divided into two portions by a well-defined ridge — the mylohyoid, or 
internal oblique ridge. It occupies a position closely corresponding to the 
external oblique ridge on the facial surface. Beginning near the base 
of the bone at the median line, it passes backward and upward, increasing 
in prominence until the base of the vertical portion of the bone is reached, 
into which it gradually disappears. This ridge gives origin to the mylo- 
hyoid muscle, which forms the central portion of the floor of the mouth. 
In correspondence to the facial surface of the bone, the attachment of the 
mylohyoideus muscle forms the dividing line between the mucous mem- 
brane and mucoperiosteum covering the upper portion of the body of 
the bone, and the periosteum covering the inferior portion. 

The Genial Tubercles. — Near the lower third, at the median line, is a 
roughened eminence — the genial tubercles. Taken collectively, these are 
in two pairs — a superior and an inferior. The superior pair (usually the 
largest) give origin to the geniohyoglossus muscle, and the lower pair to 
the geniohyoid muscle. 

The Sublingual Fossa. — By the side of the genial tubercles, and 
above the mylohyoid ridge, is a shallow, smooth depression — the sub- 
lingual fossa. One of the salivary glands — the sublingual — is partially 
supported in this fossa. 

The Digastric Fossa. — Below the mylohyoid ridge, and near the 
median line, is a slight depression — the digastric fossa — which affords 
attachment for the digastric muscle. 

The Submaxillary Fossa. — In the center of the internal surface, ex- 



54 



ANATOMY. 



tending from before backward, between the mylohyoid ridge and the 
lower border of the bone, is an oblong depression — the submaxillary 
fossa. In this fossa rests another of the salivary glands — the maxillarv. 
The Superior or Alveolar Border. — This border extends from 
the junction of the body, with the vertical plate on one side, to the corre- 
sponding point on the other. The construction of this border is similar 
to the alveolar border of the superior maxilla. At the anterior portion 

a b 




Fig. 21. — The Mandible or Inferior Maxilla. Right side, Internal Surface, a, Cofonoid 
Process; t>, Sigmoid Notch; c, Cancellated Tissue; d, Condyle; e, Ramus; /, Inferior 
Dental or Mandibular Foramen; g, Angle; h, Body; i, Internal Oblique Line. 

it is narrow, but gradually increases in width as it proceeds backward— 
in some instances following the line of the body of the bone; in others, 
inclining inward, or to the lingual. Each lateral half is marked by eight 
sockets, for the accommodation of the sixteen lower teeth. They arc- 
smaller in proportion than the alveolar sockets in the superior maxilla. 
The socket nearest the median line receives the central incisor tooth, 
and is the smallest of the number. It has an average depth of 7/16 of 
an inch, is conic from above downward, oblong in transverse section, 
with its lateral walls flattened. The second socket gives support to the 
lateral incisor tooth; it is a trifle larger than the central incisor socket, 



INFERIOR MAXILLARY BONE. 55 

but in other respects is quite similar. The socket for the cuspid 
tooth is situated at the anterior angle of this border, and is much 
larger and deeper than the incisor sockets. It has an average depth of 
9/16 of an inch; its lateral walls are compressed, and sometimes slightly 
concave. In transverse section the labial wall forms a larger curve than 
the internal or lingual wall. Passing backward, the next two sockets are 
for the support of the bicuspids; they are circular in outline, with an aver- 
age depth of 1/2 of an inch. The cavity for the first bicuspid is usually 
a little larger than that for the second. In rare instances one or the other 
of these sockets will be divided for the accommodation of two roots. 
The sockets for the first and second molars present a circular outline 
upon their free margins, but below they divide into two flattened, cone- 
shaped cavities — one anterior and one posterior. The flattened sides of 
these cavities are concave in the center, and at their lower third curve 
backward. The average depth of these sockets is 1/2 of an inch. The 
socket for the third molar, like its superior fellow, is variable both in form 
and position, frequently being crowded inside or outside of the tooth-line. 
In some instances it is divided into two or more compartments. The 
average depth is not over 3/8 of an inch. 

The alveolar process, which composes the superior border of the 
body of the mandible, differs from the same process in the superior max- 
illa in one very important particular: instead of the outer plate being 
thin and frail, it is equally as heavy as the inner or lingual plate. When 
the tooth-line is inclined inward from the body of the bone, the posterior 
outer wall is much heavier than the interior. 

The Inferior Border of the Body of the Bone. — This border extends 
from a slight depression, to be observed at the point of union between 
the body and ramus, to the corresponding point upon the opposite side. 
It is strong, rounded, and compact, and gives to the bone the greatest 
portion of its strength. Near its junction with the ramus is the facial 
notch, so named from the facial artery passing over this point. 

The Ramus, or Vertical Portion of the Bone. — This vertical 
plate is quadrilateral in outline, and presents two surfaces — external 
and internal; four borders — superior, inferior, anterior, and posterior; 
and two processes — the condyloid and the coronoid. 

The external surface is flat and smooth. Near the center it is slightly 
concave and roughened for the attachment of one of the muscles of mas- 
tication — the masseter. 

The internal surface presents near the center an oblong opening — 
the inferior dental or mandibular foramen — leading into the inferior dental 



56 ANATOMY. 

or mandibular canal. Surrounding this foramen, on its posterior inter- 
nal margin, is the mandibular spine, to which is attached the spheno- 
mandibular ligament. Running obliquely downward from the base of 
the foramen, and beneath the spine, is a decided groove — the mylohyoid 
groove — which accommodates the mylohyoid nerve, artery, and vein, 
which pass forward to supply the floor of the mouth. Below and behind 
this groove the surface is roughened for the attachment of another muscle 
of mastication — the internal pterygoid. 

The Inferior Dental or Mandibular Canal. — Beginning at the foramen 
of the same name, this canal enters the body of the bone, passes downward 
and forward horizontally, until it finds an exit at the mental foramen. 
This canal lies immediately below the alveolar sockets, and from it are 
given off smaller canals which open into the tooth-sockets through 
minute foramina. Near the mental foramen the canal divides into a 
number of smaller ones, which pass forward through the substance of the 
bone to the sockets of the cuspid and incisor teeth. 

The Superior Border of the Ramus. — This border is crescent-shaped, 
and is otherwise known as the sigmoid notch. Arising from its anterior 
portion is a flattened, cone-shaped process — the coronoid process. On 
its posterior portion is a rounded or oblong eminence — the condyloid 
process. The concave or crescent-shaped margin of this border is thin 
and smooth in front, becoming wider and heavier as it approaches the 
condyle. 

The Coronoid Process. — The anterior margin of this process, being 
a continuation of the external oblique line, is heavier at the base than at 
the apex. The outer surface is smooth, and affords attachment to the 
masseter, and a few fibers of the temporal muscle. The internal surface 
is marked by a vertical ridge, which passes downward, increasing in size, 
and finally joining the internal oblique line at a point posterior to the 
third molar. The surface anterior to this ridge is grooved, and gives 
attachment to a part of the temporal muscle above, and the buccinator 
muscle below. The surface posterior to this ridge affords attachment 
for the greater part of the temporal muscle. The posterior border of 
this surface is thin, and forms the anterior margin of the sigmoid notch. 

The Condyloid Process. — This may be described as the expanded 
extremity of the posterior border of the ramus, and is quite variable in 
form (see Occlusion of the Teeth). It is divided into -a superior or 
articular portion, and an inferior portion, or neck. 

The articular portion of the condyle is more or less oblong, and i> 
convex above, fitting into the glenoid fossa of the temporal bone, and 



INFERIOR MAXILLARY BONE. 57 

forming, with the interarticular cartilage which lies between the two sur- 
faces, the temporomaxillary articulation. 

The neck is that constricted portion immediately below the articular 
surface. It is flattened in front and presents a pit — the pterygoid fossa — 
to which a portion of the pterygoid muscle is attached. Immediately 
below the point of junction between the neck and the articular surface 
externally is the condyloid tubercle, to which is attached the external 
lateral ligament. 

The Inferior Border of the Ramus. — This border is thick, rounded, and 
continuous with the lower border of the body of the bone. At the point 
of junction between this and the posterior border is the angle of the jaw. 
The angle has a slight outward inclination, and is roughened for the 
attachment of a part of the superficial portion of the masseter muscle. 

The anterior border has been described in connection with the coro- 
noid process. 

The posterior border is smooth and rounded on its upper half, the 
lower half being roughened for the attachment of the stylomaxillary 
ligament. 

Attachment of Muscles. — The following muscles are attached 
to the mandible: 

Buccinator, Superior constrictor of pharynx, 

Depressor labii inferioris, Masseter, 

Depressor anguli inferioris, Orbicularis oris, 

Levator menti, Internal and external pterygoid, 

Geniohyoglossus, Geniohyoid, 

Platysma myoides, Mylohyoid, 

Digastric, Temporal. 

Development.* — On account of its early functional activity, the 
mandible is among the first bones to ossify. Development takes place 
from six centers for each lateral half, the nuclei being deposited as early 
as the sixth or eighth fetal week, and after their establishment the develop- 
mental process takes place very rapidly. The six centers of ossification 
are principally named according to their position. The early preparation 
for the development of the bone is found in the appearance of what is 
known as the mandibular plates, which are thrown out from the sides 
of the cranial base, and finally unite at the median line. Not long after 
this period a cartilaginous band — Meckel's cartilage — is developed in the 
substance of the mandibular plates, and it is about this cartilaginous 

* See " Development of the Teeth." 



58 



ANATOMY. 



framework that ossification first takes place. The various centers are 
distributed along the line of Meckel's cartilage, and are named as follows: 
Mental, dentinary, coronoid, condyloid, angular, and splenic. The 
mental center provides for the development of that portion of the bone 
between the median line and the mental foramen. The dentinary center 




Childhood 



Adult 



Senile 



Fig. 22. — Chart Showing the Evolution and Degeneracy of the Mandible. 



forms the lower border and outer plate, and provides for the establishment 
of the crypts inclosing the developing tooth-follicles. 

The coronoid and condyloid centers are both instrumental in con- 
structing these processes, and the angular center provides for the angle of 
the bone. The splenic center is somewhat later in making its appearance, 
and from it the inner plate of the mandible is formed, the line of union 



THE HYOID BONE. 59 

between it and the dentinary center being indicated by the mylohyoid 
groove. While, as above stated, most of the centers of development are 
along or near the line of Meckel's cartilage, the condyloid and coronoid 
processes are developed from other cartilage. Soon after birth the two 
lateral halves of the mandible begin to coalesce at the median line, this 
union taking place from below upward; and by the eighth or tenth month 
union is complete and the individual bone is established. The inferior 
maxilla is subject to a continuous change in form, not only in regard to 
its general contour, but also accommodating itself to the movements and 
growth of the teeth, the former taking place at or about the angle, while 
the latter occurs in the alveolar portion of the bone. 

Figure 22 represents the changes which take place in the angle of 
the mandible from youth to old age. It will be observed that the angle 
formed in the adult bone, with the teeth in position, is almost a right 
angle; and that in youth, with the deciduous teeth in the alveoli, the angle 
is much more obtuse, which condition is again approached in old age. 

THE HYOID BONE. 

This is a U-shaped bone placed in the upper part of the neck at the 
median line near the base of the tongue. It has no bony connection 
with other bones; it is classed as a floating bone. It is made up of a body 
and four processes. 

The body, or central portion, is quadrilateral in outline, somewhat ob- 
long from side to side, with its anterior aspect convex, and presents a 
longitudinal ridge which divides it into a superior and an inferior portion. 
It is also usually divided at the median line by a slight vertical ridge. 
At the point of junction between the longitudinal and vertical ridges a 
slight tubercle is formed. The anterior surface is given up to the attach- 
ment of muscles. The posterior surface of the body of the bone is con- 
cave and smooth, and is directed backward and downward. The superior 
border gives attachment to the thyrohyoid membrane, while the inferior 
border, which is somewhat thicker, gives attachment to the sternohyoid 
and thyrohyoid muscles. 

The processes known as the greater and lesser cornua are four in num- 
ber, one of each kind on either side. 

The greater cornua project backward and upward, and their lower 
borders and anterior surfaces are occupied with muscles. The thyro- 
hyoid ligament is attached to the posterior terminal corner. 

The lesser cornua are short conical pieces of bone, and project upward 



60 ANATOMY. 

and backward from the extremities of the body of the bone. They 
give attachment to the stylohyoid ligaments. 

Development. — Ossification takes place from five centers, one for 
the body of the bone and one for each cornua. 

Attachment of Muscles. — 

Geniohyoglossus, Sternohyoid, 

Geniohyoid, Lingualis, 

Thyrohyoid, Omohyoid, 

Mylohyoid, Digastric, 

Hyoglossus, Middle constrictor. 

The thyrohyoid ligament, as well as the stylohyoid, and the thyrohyoid 
membrane are also attached to this bone. 



CHAPTER III. 

The Temporomandibular Articulation. — The Muscles of 

Mastication. 

TEMPOROMANDIBULAR ARTICULATION. 

Although external to the cavity of the mouth, this articulation is so 
closely associated with the masticatory function that it seems important 
that a brief description of its construction and action should be given. 
It receives its name from the two bones which enter into its formation — 
the temporal and the mandible, or inferior maxillary. 



Condyle 



Ramus of 
Mandible 




Glenoid 
Fossa 



Coronoid 
Process 



Fig. 



-Temporomandibular Articulation. 



This joint is the seat of motion in the mandible, and entering into its 
construction are bones, ligaments, cartilage, and synovial membrane, 
these being the tissues essential to all diarthrodial or movable articulations. 
The various movable joints of the body are classified according to the 

6r 



62 ANATOMY. 

nature of the movement, and correspond to the mechanical actions known 
as hinge joint, ball-and-socket joint, gliding joint, pulley joint, etc. The 
temporomandibular joint is of the diarthrodial class, and the movements 
which it possesses are a combination of the gliding movement (arthrodia) 
and of the hinge movement (ginglymus). The osseous parts entering 
into the formation of the joint are the anterior portion of the glenoid 
fossa of the temporal bone and the condyloid process of the mandible 
(Fig. 23). 

The glenoid fossa may be described as an oblong cavity, with its 
base directed upward, being bounded anteriorly by a heavy bone ridge 
(the anterior root of the zygoma), posteriorly by an irregular, flattened 
portion of the bone (the tympanic plate of the petrous portion), internally 
by a union of the anterior and posterior boundaries, and externally by the 
middle root of the zygoma. The floor of the fossa is traversed by a well- 
marked fissure — the glenoid fissure (fissure of Glaserius) — which divides 
the fossa into two portions, an anterior and a posterior. The anterior 
half is deeper and more concave than the posterior, and is the articulating 
portion, being occupied by the condyle, while the posterior half gives 
lodgment to the parotid gland. 

The condyloid process of the mandible having been described 
with that bone, in this connection reference will be made to the variety 
of forms which it presents, and the influence which it exerts over the 
nature of the tooth occlusion. This process, when narrow and oblong 
(Fig. 23), closely resembles the ginglymus, or hinge joint, and will be 
accompanied by teeth presenting deep, penetrating cusps, forming a 
positive and well-locked occlusion (Fig. 24, B), with little or no lateral 
motion. If the condyle presents the appearance shown in figure 25, 
which resembles the enarthrodia, or ball-and-socket joint (although it 
cannot be considered as such), the teeth associated with such a formation 
will be provided with short, rounding cusps, and the occlusion will be 
loose and wandering (Fig. 24, A). This difference in the form of the con- 
dyle will be accompanied by a corresponding variation in the concavity 
of the glenoid fossa. Not only does the osseous structure in the joint 
partake of individual characteristics, but likewise the muscles and liga- 
ments; their functions being to operate the articulation, they are developed 
in accordance with the action required of them, which action is, in a 
measure, dependent upon the conditions existing in the mouth. 

Both the condyle and the glenoid fossa are covered with articular car- 
tilage. In the latter this membrane extends over its anterior border, to 
facilitate the play of the joint. The condyle is held in position in the fossa 



TEMPOROMANDIBULAR ARTICULATION. 



63 



by three ligaments — the capsular, the sphenomaxillary, and stylomaxillary. 
The capsular ligament is divided into four portions — anterior and posterior, 
external and internal. The anterior portion consists of a few fibers 




Fig. 25. 

connected with the anterior margin of the fibrocartilage, attached below 
to the anterior margin of the condyle and above to the front of the glenoid 
ridge. The posterior portion is attached above just in front of the glenoid 



6 4 



ANATOMY. 



fissure, and is inserted into the posterior margin of the ramus of the 
maxilla just below the neck of the condyle. The external portion, 
otherwise known as the external ligament, is the strongest portion of the 
capsular ligament. It has a broad attachment above to the zygoma, 
from which point it passes downward and backward, and is inserted into 
the outer side of the neck of the condyle. The internal portion, or short 



Styloid Process Capsular Ligament 



Internal Lateral Ligament 
/ 




Stylohyoid Ligament 

Stylomaxillary Ligament 
Fig. 26. — Temporomaxillary Articulation — Internal View. (Denver.) 

internal lateral ligament, is composed of well defined libers, having a broad 
attachment above to the inner edge of the glenoid fossa and to the alar 
spine of the sphenoid bone; below it is inserted into the inner side of the 
neck of the condyle. 

The sphenomaxillary, or long internal lateral ligament, is a thin, 
loose band, situated some distance from the joint proper, and, as its 
name implies, has its attachment above to the alar spine of the sphenoid 



TEMPOROMANDIBULAR ARTICULATION. 



65 



bone, and also to that portion of the temporal bone contiguous to it. It 
passes downward and forward, and is inserted into the mandibular spine 
of the maxilla. 

The stylomaxillary ligament extends, from the styloid process of 
the temporal bone, downward and forward, to be inserted into the poste- 



External Lateral Ligament 



Capsular Ligament 

I 




Styloid Process 
Stylohyoid Ligament 
Stylomaxillary Ligament 

Fig. 27. — Temporomaxillary Articulation — External View. (Denver.) 

rior border of the ramus of the inferior maxilla, at a point between the 
masseter and internal pterygoid muscles. 

The inter -articular fibro cartilage is an oval sheet placed between the 
two articulating surfaces. It is thinnest at the center and becomes thicker 
as the margins of the fossa are approached, at which point it is connected 
with the fibers of the capsular ligament. Being placed immediately be- 



66 



ANATOMY. 



tween the two articular surfaces it divides the joint into two separate 
synovial cavities. Each of these synovial cavities is occupied by a syno- 
vial membrane, that occupying the upper compartment being the largest, 
and passes from the margins of the glenoid fossa above to the upper 
surface of the interarticular cartilage below. The membrane which 
occupies the lower cavity is smaller, and passes from the under surface 
of the interarticular cartilage above to the margins of the condyle below. 
The blood-supply to this articulation is derived from the temporal, middle 
meningeal, and ascending pharyngeal arteries. 

Tin- nerves are derived from the masseteric and auriculotemporal. 




Fig. 28.— Showing Variation in the Shape of the Condyles. 



The movements of this articulation present as great a range as any 
other joint in the human body. While the chief movement is of the 
ginglvmoid or hinge character, brought into play in simple depression 
and elevation of the mandible, it also has the power of extension and 
retraction, may be rotated from side to side, together with all the motions 
intermediate between these. When the mandible is depressed, the con- 
dyle moves on the fibrocartilage, and at the same time glides forward and 
slightly downward until it rests on the anterior border of the glenoid 
fossa; — this movement does not extend sufficiently to allow the condyle 
to rest upon the extreme summit of the border, except in cases of excessive 
movement, as in yawning, when the condyle may glide over the summit 



TEMPOROMANDIBULAR ARTICULATION. 



6 7 



and the joint become disarticulated. When the mandible is elevated, the 
condyle slides backward and upward, and at the same time the fibro- 
cartilage, which has extended with it, also retracts until the condyle is 
settled in the fossa. The movement of extension and retraction is by a 
horizontal gliding action, by which the mandible is thrust forward and 

Temporal Muscle 




Buccinator Muscles 
Masseteric Nerve 
Masseteric Artery 

Facial Artery 
Facial Vein 



Superficial Temporal 

Artery 
Facial Nerve 
Masseter Muscle 

Platysma Myoides 
Muscle 



Fig. 29. — Temporal Muscle. (Deaver.) 

drawn back again. In this movement, as well as in the one previously 
described, both condyles are similarly and simultaneously engaged. The 
lateral or triturating movement is made in an oblique direction. This 
consists in a rotation of the condyles within the fossae, the cartilage gliding 
obliquely forward and outward on one side, and backward and inward 
on the other, this action taking place alternately. This movement is 



68 ANATOMY. 

more or less developed in accordance with the nature of the occlusion, 
being favored by those teeth possessing but little cusp formation, with a 
consequent loose and wandering occlusion; while in that type of tooth 
associated with a long overbite and deep penetrating cusps, forming a 
firm and well-locked occlusion, this movement will be but little developed. 
If this movement be employed to throw the symphysis to one side and 
back again, and not from side to side, the condyle of that side rotates in 
the glenoid fossa, while the condyle of the opposite side is drawn forward 
and inward. 

The Muscles of Mastication. 

Occupying the back part of the side of the face, and forming an in- 
dependent group, are four muscles, usually classed as the muscles of 
mastication. While this is true to a great degree, they are not the only 
muscles brought into action during this process. They are the masseter, 
temporal, internal pterygoid, and external pterygoid. The masseter, 
temporal, and internal pterygoid lift or close the lower jaw, the principal 
function of the external pterygoid being to extend the lower jaw so that 
the lower teeth pass beyond the upper. The muscles which open the 
jaws, such as happens when the head is thrown backward, are the muscles 
of the neck. 

The masseter muscle is made up of a strong quadrate sheet, con- 
sisting of two distinct layers extending from the zygomatic arch to the 
mandible. The layers of which the muscle is composed differ somewhat 
in size as well as in the directions which they take. 

Origin. — The superficial layer, much the stronger and larger of the 
two, arises from the lower border of the malar bone, and from the anterior 
two-thirds of the zygomatic arch. The deep layer arises from the posterior 
third of the lower border and from nearly all the internal surface of the 
zygomatic arch. 

Insertion. — After passing downward and backward it is inserted into 
the outer surface of the ramus of the mandible. The deep layer, passing 
downward and slightly forward, mingles with some of the fibers of the 
superficial portion, and is finally inserted into the upper half of the ramus 
of the mandible. 

Action. — To draw slightly forward, and by its superficial layer to 
close the jaw. " In closing the jaw it acts with less mechanical disad- 
vantage than is usual with muscles. When the pressure to be overcome 
is exerted upon the back teeth, the arm of the lever upon which the power 
acts is almost as long as that which intervenes between these teeth and the 



TEMPOROMANDIBULAR ARTICULATION. 



6 9 



fulcrum. This fulcrum is not at the temporomaxillary joint, but at a 
point below the neck of the mandible, corresponding very nearly to the 
lower attachment of the internal lateral ligament. Moreover, the result- 
ant force of the muscle acting, as it does, upward and forward, is perpen- 
dicular to the lever, which may roughly be described as a bar extending 
downward and forward from the neck of the mandible to the point of the 
chin." (Morris.) 



External pterygoid 



Int3rnal pterygoid 



Interartieular flbro-cartilage 

j ■ 




Fig. }o. — The Pterygoid Muscles. (Morris.) 



Relations. — It is covered superficially by the skin and fascia of the 
platysma myoides, by the risorius and the masseter fascia, by the parotid 
gland and ducts, facial veins, and portions of the facial nerve. Deeply, the 
muscle lies in contact with the ramus of the jaw and the buccinator muscle, 
being separated from the latter by a layer of fat. A small portion of the 
temporal muscle also comes in relation to the deeper lying portion. 

The temporal muscle is covered by a strong membrane, the temporal 
fascia, which arises from the temporal ridge, and is inserted into both the 
inner and outer portions of the upper border of the zygomatic arch. Near 
this point it divides into two distinct layers, one passing to the inner, the 
other to the outer margins of the zygoma. Below the fascia is continuous 



70 ANATOMY. 



with the masseteric fascia. Passing downward from this to the inferior 
borders of the ramus of the jaw, it envelops the masseter muscle. The 
muscle itself is radiating and fan-shaped in form, located in the temporal 
fossa, from which point it descends to the coronoid process of the 
mandible. 

Origin. — From nearly the entire surface of the temporal fossa, from 
the temporal ridge, from the entire surface of the temporal fascia, 
down to its lower attachment to the zygomatic process. 

Insertion. — -Into the coronoid process of the mandible. 

Action. — To close the lower jaw, some of its fibers drawing the jaw 
backward after the other muscles have protruded it. 

Relations. — Its superficial portion is covered by the temporal fascia 
which separates it from some of the auricular muscles; branches of the 
facial nerve, the auriculotemporal nerve, and a portion of the epicranial 
aponeurosis. The temporal fossa and the external pterygoid muscles 
are in relation with it deeply. 

The internal pterygoid muscle is a thick, quadrilateral, sheet- 
like muscle, and receives its name from its origin and relative position. 

Origin. — From the inner surface of the external pterygoid plate; the 
tuberosity of the palate bone and a small portion of the maxilla. 

Insertion. — Into the internal surface of the ramus of the mandible at 
its lower and posterior borders and extending as high as the mandibular 
foramen and mylohyoid. 

Action. — To close the jaw and at the same time draw it backward and 
throw it toward the opposite side. "The same remarks which were 
made with respect to the very small loss of mechanical advantage in the 
masseter muscle apply to this muscle. When closed it will draw the 
jaw forward; and also it will help the external pterygoid in drawing the 
ramus of its own side toward the middle line." (Morris.) 

Relations. — Superficially, the internal maxillary vessels, the external 
pterygoid muscle, the internal lateral ligament, the inferior dental and 
lingual nerves. Deeply, the submaxillary glands; the tensor palati and 
superior constrictor of the pharynx, as well as the stylohyoid and posterior 
border of the digastric muscles. 

The external pterygoid muscle is composed of two triangular sheets, 
one passing in a horizontal and the other in a vertical direction. It re- 
ceives its name from its attachment to the pterygoid process of the 
sphenoid bone as well as its relation to its companion muscle, the internal 
pterygoid. 

Origin. — It is composed of two distinct heads, an upper and a lower. 



TEMPOROMANDIBULAR ARTICULATION. 7 1 

The upper head arises from the greater wing of the sphenoid bone, from 
the internal pterygoid ridge, and external to the foramen ovale and fora- 
men spinosum. The lower head arises from the outer surface of the ex- 
ternal pterygoid plate. 

Insertion. — The upper head is inserted into the inter-articular fibro- 
cartilage, into the capsule of the joint as well as the neck of the condyle. 
The lower head is inserted into the neck of the condyle. 

Action. — To draw the condyle and inter-articular fibrocartilage for- 
ward and inward. "The combination of these two movements produces 
the oblique movement of the lower molar teeth of one side, forward and in- 
ward with respect to the upper molars which are their opponents. It 
should be observed also that this inward movement of one side is the 
agent by which the ramus of the opposite side is moved outward. To 
assist in opening the mouth by depression of the lower jaw. As the 
transverse axis of this movement passes through the mandible at two 
points situated below the necks of the rami, it follows that a forward 
movement of the condyles and necks will assist in the backward move- 
ment of the angles and body which accompanies the depression of the 
mandible." (Morris). 

Relations. — Superficially, some of the fibers of the internal pterygoid, 
the temporal, and part of the masseter muscle. Deeply, the internal 
pterygoid muscle, the middle meningeal and inferior dental vessels, in- 
ternal maxillary vessels, and masseteric and posterior deep temporal 
nerves passing behind or through the attachment to the upper head; the 
inferior dental and lingual gustatory nerves beneath the lower head. 



CHAPTER IV. 

A General Description of the Teeth. — The Permanent Teeth: 
Classification, Surfaces, Etc. — The Roots of the Teeth. 
The Dental Arch. 

A GENERAL DESCRIPTION OF THE TEETH. 



Apex 



A Tooth (Fig. 31). — One of the thirty-two specialized organs for the 
seizure and mastication of food, placed at the entrance to the alimentary 
canal (the mouth). The typical form of a tooth is a modified cone or 

combination of cones, and is composed of 
two fundamental parts — the crown and the 
root or roots. The crown is that part which 
projects beyond the gum and is visible in 
the mouth; while the root is that part which 
is implanted in the bone and covered by 
the gum. Intervening between these two 
extremities, and usually occupying a por- 
tion of each, is a third division — the neck. 

Completely covering the crown of a 
tooth is a hard, vitreous-like substance, 
enamel;* the root is covered by a hard, 
bone-like substance, cementum;* while the 
interior or body of the organ is composed 
of a hard substance closely resembling bone, 
the dentin.* The neck of a tooth, which 
serves to unite the crown to the root, and 
which is usually formed at the expense of 
each, is covered partly by enamel and partly 
by cementum. This brief description in the singular number can best be 
continued in the plural. Teeth are classified according to their form, which 
is always in accordance with their function, into simple and complex. In 
the simple class the single mod'fied cone is the predominating form, the free 
extremity of the crown serving as the base of the cone, while the apex i- 
formed by the free end of the root. Included in this same classification 







Fig. ,}i 



*See Tissues of the Teeth, Pari II. 



A GENERAL DESCRIPTION OF THE TEETH. 73 

are those teeth which are made up of a double cone, or a simple cone and 
an inverted cone attached to each other at a common base (Fig. 31). 
The purposes for which such teeth are adapted are those of grasping, 
incising, and tearing, and they are usually so arranged that the free ex- 
tremities of their crowns interlock or overhang the opposing teeth in the 
opposite jaw. 

In the complex class (Fig. 32) the external form of the tooth is pro- 
duced by a combination of cones, some of which are simple, others inverted, 
but all uniting at a common base — the neck 
of the tooth. In this class the simple cones 
form the roots of the tooth, while the crowns 
are made up of a number of smaller cones, 
much modified. Such teeth are adapted to 
crushing and grinding, and are less inclined 
to interlock during active service. 

The teeth are divided into two grand 
divisions — those of infancy and childhood, 
called deciduous or temporary teeth, and those 
of the adult period, known as permanent teeth. 
The latter class being most important, will 
first receive consideration. 

The Permanent Teeth (Fig. 33).— The 
permanent teeth, thirty-two in number, are divided into those of the 
superior portion of the mouth, upper, and those of the inferior portion, 
lower. In number they are equally divided, each jaw giving support to 
sixteen. They are firmly imbedded in the alveolar sockets of three of 
the bones of the mouth, the upper sixteen being attached to the two 
superior maxillary or upper jaw-bones, and the lower sixteen to the 
mandible, inferior maxillary, or lower jaw-bone. As above referred to. 
the attachment of the teeth to the bones is by implantation in sockets, 
the alveoli (see description, " Bones of the Mouth"). In this attachment 
there is a special development of bone, closely modeled to the roots of 
the teeth, and which is subservient to the ever-varying changes which 
take place during the development of the organs. The joint thus 
formed between the roots of the teeth and the alveoli is of the immovable 
or synarthrodial class, and is styled gomphosis. Intervening between the 
roots of the teeth and the walls of the alveoli is a delicate membrane 
the alveolodental membrane. 

Before continuing the description of the teeth, a further classification, 
which refers alike to the upper and lower organs, must be presented. 




Fig. 32. 



74 



ANATOMY. 



This classification is derived from the function and form of the teeth. 
Figure 33 shows the thirty-two teeth removed from the jaws and placed 
side by side in two straight lines. In the center is a perpendicular line, 
which corresponds to the median line or center of the mouth, the teeth 
at either extremity being those which occupy the back part of the mouth. 
Without confining the description to either the upper or lower teeth, it will 
be observed that the first two teeth upon either side "of the median are 
similarly formed, and all four are called incisors (incidere, to cut); the 
two larger incisors, being nearest the median line or center, are called 



Lingual Surfaces 



Labial and Buccal Surfaces 



Uppe 



Lower 




Lingual Surfaces 

Fig. 35 



Labial and Buccal Surfaces 

-The Permanent Teeth. 



central incisors; while the two smaller being placed at the side of the cen- 
trals, are known as lateral incisors {lateralis, the side). The third tooth 
from the median line upon either side is the cuspid (cuspis, a point), so 
named from possessing a single cusp or point. Passing to the right or left 
on the chart, or backward in the mouth, the fourth and fifth teeth from 
the median line are the bicuspids (bi, two; cuspis, a point), having two 
points or cusps. The bicuspid nearest the median line is the first 
bicuspid; that most distant from the median line is the second bicuspid. 
The sixth, seventh, and eighth teeth from the median line upon either 
side are those of another class, the molars (mola, a mill-stone), being 
named according to their function, that of crushing or grinding the 
food. Proceeding from before backward, the molars are denominated 



A GENERAL DESCRIPTION OF THE TEETH. 75 

first molar, second molar, and third molar. To sum up, the names and 
number of the permanent teeth may be given by the dental formula, as 
follows : 

Incisors, \ Bicuspids, \ 

Cuspids, i Molars, § 

The Surfaces of the Teeth. — The crown of each tooth presents 
five surfaces, which are variously named, in accordance with the duty 
which they are called upon to perform or suggestive of their location. 
The outer surface of the incisors and cuspids, or that contiguous to the 
lips (labia), is called the labial surface; the corresponding surface of the 
bicuspids and molars, or that contiguous to the cheeks (buccae), is the 
buccal surface. That surface of both upper and lower teeth which faces 
the palate or tongue is characterized as the lingual surface. 

The proximate surfaces of the teeth are named with regard to their 
relation to the median line, those surfaces nearest to this point being 
called mesial, those most distant, distal. In addition to these four sur- 
faces, which represent what might be termed the sides of the teeth, a fifth 
surface is present, that which occludes with the teeth of the opposite jaw, 
and is called the occlusal surface. 

In the incisors and cuspids this surface is formed by the converging 
of the labial and lingual surfaces, forming an edge to the free extremity of 
the crown, named, from its action in mastication, the incisive or cutting- 
edge. In the bicuspids and molars the various sides of the crowns 
remain nearly parallel to each other throughout their extent, thus provid- 
ing a surface nearly equal to, or greater than, any of the others, and one 
well adapted to the purposes for which it is intended — that of grinding or 
crushing the food. 

The Roots of the Teeth. — The upper incisors and cuspids are each 
provided with one root; the upper first bicuspid may have one or two 
roots, most frequently the latter; while in the second bicuspid a single root 
is usually present. The upper first and second molars are each supported 
in the jaw by three roots, and while in the upper third molar three roots 
are most common, the number is quite variable, ranging from a single 
cone-shaped root to three, four, five, or even six smaller branches given off 
from a common base. 

In the lower incisors, cuspids, and bicuspids, a single root is most 
frequently met with, although the latter class, in rare instances, may be 
provided with two. The lower first and second molars are each provided 
with two roots, but in the third molar, like its upper fellow, the number 



7 6 



ANATOMY. 




may be diminished or increased. In the upper molars, two of the three 
roots are placed above the buccal half of the crown, and are called buccal 
roots; the remaining root is placed above the lingual half of the crown, 
and is designated as the lingual root. In the lower molars, one of the 
two roots is placed below the anterior or mesial half of the crown, and is 
named the mesial root, and the other below the posterior or distal half, 
and is known as the distal root. In those teeth with a complicated root 

formation, it would seem to be a 
question whether they are pos- 
sessed of a single root, with two 
or more branches, or separate 
and distinct roots throughout. 
To determine this, some account 
must be taken of the point at 
which the bifurcation or trifurca- 
tion takes place. If this separa- 
tion be in close proximity to the 
crown, the tooth should be con- 
sidered as having more than one 
root (Fig. 34, A) ; but, on the 
other hand, if the point of sepa- 
ration be some distance from the 
crown, with a solid mass of root substance intervening, the tooth may 
be said to possess a single root, with two or more branches (Fig. 34, B). 
In the latter instance, that part of the tooth between the point of 
reparation and the crown is called the root or root base; while the 
prolongations beyond the point of separation are known as the branches 
of the root. 

The roots of the teeth are not only variable in number, but are also 
subject to much variety in form. In the anterior teeth (the incisors and 
cuspids) the roots are inclined to the form of the simple cone, which form, 
however, is frecpjently more or less broken by a slight curvature near their 
extremities, or by a slight compression of their lateral walls. In the poste- 
rior teeth (the bicuspids and molars) the roots, root bases, or root branches 
are all inclined to the conical form, but do not approach so nearly the 
perfect cone as those of the anterior teeth. These roots are also more or 
less crooked and flattened laterally. The free extremity of the roots 
of the teeth, forming as they do the apex of these cone-like prolongations 
of the crowns, are known as the apices or apical extremities. 

The extent of the enamel covering to the crowns of the teeth is 



Fig. 34. 



A GENERAL DESCRIPTION OF THE TEETH. 



77 



marked by a well-defined line, which completely encircles the neck of the 
tooth, the cervical line. 

The Dental Arch (Fig. 35). — The teeth are arranged in the jaws 
in the form of two parabolic curves, the superior arch describing the 
segment of a larger circle than the inferior, as a result of which the upper 
teeth slightly overhang the lower. Figure 35 represents the sixteen 
upper teeth in position in the bone, presenting their occlusal surfaces, a 
part of their lingual surfaces also being visible. Viewed in this direction, 
the gradual change in the crowns of the teeth from the simple incisors to 



Incisors 



Cuspid 




3d Molar 



Fig. 35. — The Dental Arch. 



the complex molars may be observed. An examination of the central 
incisors will show how perfectly they are adapted to the process of cutting 
or incising the food, the cutting-edge being sharp and the lingual surface 
comparatively smooth and unbroken. In the lateral incisors the cutting 
feature predominates, but the lingual surface is broken near the neck of 
the tooth by a slight depression, surmounted by a more or less pronounced 
fold of enamel, in many instances resembling a small cusp. The crown 
of the cuspid tooth furnishes the intermediate form between the simple 
and the complex. This tooth, instead of being provided with a straight 
cutting-edge, is surmounted at the center of its occlusal surface with a 
well-defined point or cusp, descending from the summit of which are two 



78 ANATOMY. 

cutting-edges, one passing to the mesial and one to the distal. The 
lingual surface of this tooth presents a marked contrast to the correspond- 
ing surface of the incisors, being broad and full, and frequently provided 
with a prominent ridge of enamel in the region of the neck, showing a 
rapid approach to the complex form. In the bicuspids the buccal half 
of the crown is quite similar to the crown of the cuspid, but in the lingual 
half a complete revolution has taken place. The enamel fold — but slightly 
apparent in the incisors, and somewhat increased in the cuspids — has 
now become a fully developed cusp, resulting in the production of an 
occlusal surface adapted to crushing or grinding, instead of incising or 
tearing. In the molars, the increase in the size of the tooth-crown is ac- 
companied with an occlusal surface much more complex than any of 
the teeth previously described, and one well adapted to its function- 
that of crushing and grinding the food. 

Arrangements of the Teeth in the Dental Arch (Fig. 35). — Beginning 
with the upper teeth, the central incisors are found occupying the center 
of the arch, and are, therefore, slightly in advance of the laterals. These 
teeth are so implanted in the alveoli that their crowns are not perpendic- 
ular, the cutting edge being slightly more prominent than the neck of 
the tooth. The roots are also somewhat inclined from the median line, 
and as a result the crowns have a slight mesial inclination, the mesial sur- 
faces approximating each other at or near the cutting-edge, with a slight 
space intervening at the necks. In certain typal forms — the bilious, for 
example — when the front of the arch is flat, the labial surfaces of these 
teeth form nearly a direct line from side to s.'de; while in those types in which 
the arch is well rounded anteriorly, notably in the sanguine temperament, 
the labial surfaces of these two teeth form a small segment of the arch, 
so that the mesial extremity of the cutting-edge of each tooth-crown is 
somewhat in advance of the distal. The lateral incisors are similarly 
implanted in the alveoli, causing their cutting-edges to project. The 
roots of the lateral incisors usually have a stronger distal inclination than 
those of the centrals, and the crowns show a more marked mesial inclina- 
tion. The mesial surfaces approximate the distal surfaces of the central • 
incisors at or near the cutting-edge. When the front of the arch is flattened, 
these teeth are but little less prominent than the central incisors, but when 
the arch is well rounded they continue the segment begun by the centrals, 
and are necessarily less prominent. While the occlusal surfaces of the 
teeth are usually considered as forming a perfect plane (see Occlusion of 
the Teeth), the lateral incisors are generally a trifle shorter than the 
centrals. The cuspids may be considered as occupying the corners or 



A GENERAL DESCRIPTION OF THE TEETH. 79 

turning-points of the arch. They are more prominently placed than the 
adjoining teeth, this feature being increased by the bulging or general 
convexity of their labial surfaces. The extremity of the occlusal surface 
of the cuspids — i. e., the point of the cusp — is a trifle below the cutting- 
edge of the laterals and about on a line with that of the centrals. While 
the apical extremities of the roots of the cuspid teeth are directed away 
from the median line, the crowns assume 'almost a perpendicular, this 
condition resulting from a bend in the tooth at the neck. Although the 
perpendicular position is most commonly assumed by the crown, it is not 
unusual to find either a mesial or distal inclination present. Reference 
has been made to the cuspid teeth occupying a position which might be 
termed the turning-points or corners of the arch, and in most instances 
it may be thus considered; but in certain typal forms — the sanguine, for 
example — the tooth-line is unbroken and passes over the cutting-edges 
of the incisors, the summit of the cusps of the cuspids, and is continued 
backward over the buccal cusps of the posterior teeth. The bicuspids 
are placed nearly perpendicular in the arch, but occasionally deviate 
from this by a slight mesial or buccal inclination. The length usually 
corresponds to that of the central incisors, and their buccal surfaces are 
slightly less prominent than the corresponding surfaces of the cuspid 
teeth. The increase in the buccolingual diameter of the crowns of 
the bicuspids over that of the incisors and cuspids results in breaking 
the lingual line of the occlusal surfaces. In the bilious and kindred 
types, the tooth-line is carried directly backward from the cuspid to 
the first molar, making the buccal face of the bicuspids equally promi- 
nent, but when the arch is well rounded the second bicuspid is 
slightly more prominent than the first. The first and second molars 
usually assume a perpendicular position, but are occasionally inclined to 
the distal and buccal. The relative prominence of the buccal as well as 
the lingual surfaces of these teeth is also controlled by the form of the arch. 
The occlusal surfaces are about on a level with those of the bicuspids 
and central incisors, but generally the lack of development in the distal 
half of the crown of the second molar results in the production of a slight 
upward curve to the tooth-line level at this point (see Occlusion of the 
Teeth). On account of the limited accommodations afforded it, the posi- 
tion of the upper third molar is quite variable. It may be either to the 
buccal or to the lingual of the tooth-line, and is usually strongly inclined 
to the distal. In those cases in which there is a decided dip to the arch 
(see Occlusion of the Teeth), this tooth is relatively shorter than those an- 



80 ANATOMY. 

terior to it, but when the tooth-line level is a perfect plane, the length of 
this tooth corresponds to the other molars and bicuspids. 




Fig. 36. — The Tooth-line in the Lower Jaw. 

The lower incisors are placed more nearly in a perpendicular position 
than the upper, and a reverse condition exists, in the lateral incisors 




Fig. 37. — The Tooth-line in the Lower Jaw. 



being a trifle larger than the centrals. The lower cuspids are probably 
more constant in their position than any class of teeth in the mouth, in 



A GENERAL DESCRIPTION OF THE TEETH. 



8l 



nearly all instances assuming a direct perpendicular. Like the upper cus- 
pids, they may be said to establish the corners or turning-points of the 
inferior arch, and are somewhat more prominent in the tooth-line than 




Fig. 38. — Sanguine. 



neighboring teeth. All of the six anterior lower teeth may be slightly 
inclined to the mesial. The lower bicuspids and molars, instead of 
having the buccal inclination possessed by the corresponding upper teeth, 




Fig. 39. — Bilious. 

are inclined to the lingual. The first molar seldom deviates either to the 
mesial or the distal, the second molar is generally inclined to the mesial, 
while the third molar is strongly inclined to the mesial. In the inferior 



82 



ANATOMY. 



arch the curve formed by the incisors and cuspids is the segment of a 
smaller circle than the corresponding curve in the superior arch. This 
curve may be continued over the buccal cusps of the bicuspids and molars. 




FlG. 40. -Nervous. 

or it may be broken at the cuspid tooth and continued backward in a direct 
line (Fig. 36). The teeth in the inferior arch are placed directly over the 
body of the bone as far back as the second bicuspids, while the molars 





» i ~ r . 




• « 




^^\ 


' 1 








^^^^^^^B 




*Jm 






vT 




T-it 


§1 




G? 



Fig. 41. — Lymphatic. 

frequently overhang the body of the bone by an extension of the alveoli 
inward (Fig. 37). 

The curve described by the dental arch is quite variable, and this 



A GENERAL DESCRIPTION OF THE TEETH. 83 

variation is generally referred to in connection with the temperament. 
Thus, in the sanguine temperament (Fig. 38), the arch is well rounded 
anteriorly, the circle being continued backward to the region of the molars, 
where the line is broken by slightly inclining to the lingual. In this 
arch the distance in a straight line from the center of the second molar, 
on one side, to the center of the corresponding tooth on the other, is about 
equal to the distance from either of these points to the median line be- 
tween the central incisors, forming a right-angle triangle. In the bilious 
temperament (Fig. 39) the arch presents a broad front from cuspid to 




Fig. 42. — Section of the Superior Maxilla Showing Interproximate Spaces. 

cuspid, with but little curve; at these points it turns abruptly backward, 
being continued almost in a direct line to its extremity. In this arch 
the side of the triangle (represented by the line from molar to molar) 
is much reduced in length. In the nervous temperament (Fig. 40) 
the arch is Gothic in form, the segment formed by the anterior teeth 
being that of a much smaller circle than either of the types previously 
referred to. The distance from molar to molar is much less than the 
distance from molar to median line. In the lymphatic temperament 
(Fig. 41) the arch is well rounded and broad, the segment being that 
of a much larger circle than any of the above, the side of the triangle 
formed by the line from molar to molar being of the greatest length. 

Interproximate Spaces. — In the mesiodistal direction the crowns 
of the teeth, as a class, are broader at their occlusal surfaces or cutting- 
edges than at their necks (Fig. 42). This bell-shaped form of the tooth- 
crowns causes their proximate surfaces to touch at a point representing 



84 ANATOMY. 

their greatest mesiodistal diameter, which is usually near the cutting-edge 
or occlusal surface. Between this point of contact and the cervical line 
there exists a V-shaped space, called the interproximate space. These 
spaces are largest in that class of teeth found in the nervous and bilious 
types, where the necks of the teeth are much constricted, and the bell- 
shaped crown strongly outlined. In teeth of this class the point of contact 
is slight, and the interproximate spaces are only partially occupied by 
the gum tissue, leaving a free passage between the point of contact and 
the gingival margins. In the sanguine and lymphatic temperaments the 
proximate surfaces of the teeth are nearer parallel with one another, thus 
making the point of contact cover a greater extent of surface, and reducing 
the size of the interproximate spaces. 



CHAPTER V. 

Occlusion of the Teeth. 

As stated elsewhere, the teeth are arranged in the mouth in the form 
of two parabolic curves, one of which occupies the upper half and the 
other the lower half of the cavity. To properly perform their function it 
is necessary for the upper and lower teeth to come into contact, which 




Fig. 43. — The Teeth in Occlusion. 

they are enabled to do by the movement of the lower jaw, and it is the 
relation existing between the two when thus brought together that con- 
stitutes the occlusion of the teeth. During mastication the teeth do not 
only occlude, and remain stationary at a given point until the lower jaw 
is again depressed, but, through the combined movements of the man- 
dible, the lower teeth are made to move from side to side, thus grinding 
or crushing any substance placed between the occlusal surfaces of the 
bicuspids and molars. This gliding antagonism of the teeth is commonly 
termed the articulation, and it is important that a distinction be made be- 

85 



86 ANATOMY. 

tween the terms "occlusion" and "articulation," the former referring to 
the relations existing between the upper and lower teeth when brought 
together normally and held firmly in that position, while the latter relates 
to the various movements of the teeth after being brought together in 
occlusion. In the majority of instances the segment described by the 
superior arch is somewhat larger than that formed by the inferior, and 
the upper teeth project over and are partly outside of those in the inferior 
arch. Figure 43 presents a labial and buccal view of the teeth in position 
in the alveoli, and also in occlusion. It will be observed that the upper 
teeth are not directly antagonistic to those of the same name in the in- 
ferior arch. There are two reasons for the presence of this condition: 
First, the mesiodistal diameter of the upper central incisors is much 
greater than that of the corresponding lower teeth; second, the larger 
circle present in the superior arch. This arrangement provides that 
each tooth, instead of being antagonized by a single tooth of the opposite 
jaw, is met in occlusion by a portion of two teeth. The upper central 
incisor is met in occlusion by the entire cutting-edge of the lower central 
incisor and the mesial third of the cutting-edge of the lower lateral in- 
cisor. The upper lateral incisor is met in occlusion by the distal two-thirds 
of the cutting-edge of the lower lateral incisor and by the mesial cut- 
ting-edge of the lower cuspid. The upper cuspid is met in occlusion by 
the distal cutting-edge of the lower cuspid and by the mesial two-thirds 
of the buccal cusp of the lower first bicuspid. The upper first bicuspid 
is met in occlusion by the remaining or distal third of the lower first 
bicuspid and by the mesial two-thirds of the buccal cusp of the lower 
second bicuspid. The upper second bicuspid is met in occlusion 
by the remaining or distal third of the buccal cusp of the lower second 
bicuspid, and by the mesial incline of the mesiobuccal cusp of the lower 
first molar. The upper first molar is met in occlusion by the distal in- 
cline of the mesiobuccal cusp of the lower first molar, by the entire distal 
cusp of the same tooth, and by the mesial incline of the mesiobuccal cusp 
of the lower second molar. The upper second molar is met in occlusion 
by the distal incline of the mesiobuccal cusp of the lower second molar, 
by the entire distobuccal cusp of the same tooth, and by the mesial incline 
of the mesiobuccal cusp of the lower third molar. The upper third 
molar is met in occlusion by the distal incline of the mesiobuccal cusp of 
the lower third molar and by the entire distobuccal cusp of the same tooth, 
thus being the only tooth in the superior arch with but a single opponent. 
Likewise each lower tooth is met in occlusion by two in the superior arch, 
with the single exception of the central incisor, which occludes with the 



OCCLUSION OF THE TEETH. 



87 



upper central alone. There are many variations from this so-called 
typical occlusion, as above described, and any slight difference one way 
or another should not be considered abnormal. In certain types the 
segmental form of the superior arch is but little greater than that of the 
inferior, and the cutting-edges of the upper incisors occlude almost directly 
upon the cutting-edges of. the lower incisors. As a result, all of the upper 
teeth are forced to the distal, and the relationship between the upper and 
lower organs is much altered. When the upper teeth overhang the lower, 
the lingual cusps of the upper bicuspids and molars penetrate the fossae 




Fig. 44. 



-The Mandible at the Adult Period, showing the Equilateral Triangle 
described bv the Dental Arch. 



or sulci of the corresponding lower teeth, when in occlusion, and the 
buccal cusps of the lower bicuspids and molars rest in the fossae of their 
upper opponents. 

To assist in the study of the occlusion of the teeth, some reference 
must be made to the tooth-line lever, or plane of occlusion. For this 
purpose the lines forming the facial angle are of value. These lines are 
as follows: A fixed line representing the base of the angle may be drawn 
from the center of the glenoid fossa, passing forward through the anterior 
nasal spine or base of the nose (a, Fig. 45), the angle being completed by a 
perpendicular line resting upon the labial surface of the upper incisors, 
passing upward and touching the most prominent part of the forehead (b, 
Fig. 45). The tooth-line level is approximately horizontal to this basal 
line, but instead of a perfect plane we usually find the superior arch dipping 
downward, while the inferior arch will be provided with a corresponding 



88 



ANATOMY. 



depression. This dip to the arch is greatest in the region of the bicuspids, 
and the extent to which it may exist varies with the type of tooth and the 
consequent nature of the occlusion. 




Fig. 4.5. Lines showing Facia] Angle, Caucasion or White Race 

Thus far no reference has been made to what is commonly termed 
the overbite, and the cusp forms in the teeth. As these two factor exert 



/ .i* 




1 M^ A 


3^ 




• 




^v 


/ lytvor^ 




/ M^ * 





Fig. 46. — Lines showing Facial Angle, Negro or Mixed Races. 

a dominating influence over the character of the occlusion, the effects which 
they produce will be briefly described. The overbite is so named, from the 



OCCLUSION OF THE TEETH. 



8 9 



fact that the upper teeth project beyond, or overhang and partly cover the 
labial and buccal surfaces of the lower teeth. This may be a pronounced 
feature in the tooth-occlusion, or it may exist to a very slight degree. 
Although the overbite is usually referred to as existing in the incisive 
region alone, it is not confined to these teeth, but is also present in the 
bicuspids and molars by the buccal cusps of the upper teeth overhanging 




Fig. 



-Lymphatic. 



those of the lower. The extent of the overbite is gradually diminished 
from before backward, the central incisors presenting the greatest amount 
of overhanging surface, which condition is slightly decreased in the 
laterals, and a corresponding reduction is continued until the third 
molars are reached, at which point the overbite is scarcely observed. 
Where the overbite is extensive, as shown in figure 43, the upper incisors 




Fig. 48. — Sanguine. 



overhanging and hiding from view about one-third of the labial surfaces 
of the lower incisors, the cusps of the bicuspids and molars will be corre- 
spondingly long and penetrating, the buccal cusps of the upper teeth ex- 
tending well down over the buccal cusps of the corresponding lower 
teeth. In an occlusion of this class, which is usually found in the ner- 
vous and bilious types, the dip to the arch will become a prominent 



9° 



ANATOMY. 



feature, the occlusion will be firm and well locked, and the lateral articular 
movements will be slight during mastication. In the lymphatic and san- 
guine temperaments the occlusion is loose and wandering, greater freedom 
of movement being permitted by the short overbite and the corresponding 
lack of cusp-formation. Figure 47 represents such an occlusion; the 




Fig. 49. Bilious. 



superior arch is but little greater in its segmental outline than the inferior. 
The cutting-edges of the upper incisors are somewhat more prominent 
than those of the lower, but the former do not overlap the labial surfaces 
of the latter. In an occlusion of this character the dip to the arch is not 
so pronounced, and the articular movements are much more extensive. 



CHAPTER VI. 

The Blood- and Nerve-Supply to the Teeth. 

THE BLOOD-SUPPLY TO THE TEETH. 

Briefly stated, the course of the blood from the heart to the teeth is 
as follows: From the heart to the aorta, to the common carotid artery, 
to the external carotid artery, to the internal maxillary artery, from the 
various branches of which the teeth are supplied. 

The Internal Maxillary Artery (Fig. 51). — This artery, otherwise 
known as the deep facial, is the larger of the two terminal branches of 
the external carotid. In addition to supplying the teeth, it is distributed 
to the roof and floor of the mouth, to the maxillary sinus, and to other 
parts of the face and head. It has its origin from the external carotid 
artery opposite the condyle of the mandible within the substance of the 
parotid gland, passes forward between the condyle of the jaw and the 
sphenomaxillary ligament, from which point it passes obliquely upward 
and forward between the external and internal pterygoid muscles until 
it reaches the sphenomaxillary fossa, where its terminal branches are 
given off. It is divided into three portions — the first or maxillary, 
the second or pterygoid, and the third or sphenomaxillary. The teeth 
are supplied from branches of the first and third divisions, the upper 
teeth receiving their blood-supply from the alveolar or superior maxillary 
and the infra-orbital branches of the third division, while the lower 
teeth are supplied by the inferior dental or mandibular branch of the first 
division. 

The alveolar or superior maxillary branch arises, in common 
with the infra-orbital branch, from the internal maxillary as it passes into 
the sphenomaxillary fossa. It passes downward, in a tortuous manner, 
in a groove provided for it in the back of the maxilla. In its downward 
course it gives off the following branches: The antral, to supply the an- 
trum; the dental (known as the posterior dental arteries), which pass into 
the substance of the bone through the posterior dental canals to supply 
the molar and bicuspid teeth ; the alveolar or gingival, to supply the gums; 
and the buccal, to the lateral walls of the mouth. The anterior upper 
teeth are supplied through the infra-orbital branch of the internal maxil- 

9 1 




Fig. 50. — Dissection Showing Blood-supply to the Teeth. 



92 



THE BLOOD-SUPPLY TO THE TEETH. 



93 



lary\ This branch arises from the internal maxillary artery, generally, 
in common with the posterior dental. It passes forward in company 
with the maxillary division of the fifth nerve — first along the groove and 
then in the canal on the orbital plate of the maxilla, and finally makes 
its exit upon the face through the infra-orbital foramen. Besides giving 
off branches to the orbital and nasal cavities, it supplies the incisor and 
cuspid teeth through its anterior dental branch, which passes downward 
through a groove in the anterior wall of the maxilla. 



Infraorbital artery and nerve 



Spheno-pa/afine branch 

Posterior or descending palatine branch 

Auso-j.uiattnc brunt h 

Vidian branch 

interior deep temporal artery 
External pterygoid branch 



Orbital branch 

Palpebral 

branch 



Xusat branch 

Anterior 

dental branch 

Labial branch 

Posterior dental 

branch 



Alveolar branch 



Incisive branch 
Mental branch 

Submental branch 




Posterior deep temporal inter)/ 

Small meningeal 
artery 

Middle meningeal 
artery 



Temporal ariern 
Tom panic branch 

Deep auricular 

brunch 
A URICULO-TE3I- 
PORAL KERVE 

Masseteric branch 



External carotid 
a iter if 



Interned lateral or 
spheno-mandibu- 
lar ligament 

Mnndihulnr or 
inferior dentul 
artery and nerve 



Buccal branch with Internal pterygoid branch 

portion of buccal nerve 

Mylo-hyoidean branch 
Fig. 51. — Scheme of Internal Maxillary Artery. (Morris.) 



The Inferior Dental Artery (Fig. 51). — The lower teeth receive 
their blood-supply through the inferior dental or mandibular artery. 
This artery arises from the under part of the internal maxillary as it 
passes downward and forward between the sphenomaxillary ligament 
and the neck of the jaw and enters the inferior dental canal through the 
inferior dental foramen. It passes forward in the canal accompanied 
by the inferior dental nerve, and in so doing sends off twigs to supply 




Fig. 52. — Dissection Showing Nerve-supply to the Teeth. 



94 



THE NERVE-SUPPLY TO THE TEETH. 95 

the molar and bicuspid teeth. When the mental foramen is reached, 
it divides into two branches, the incisive branch and the mental branch. 

The incisive branch continues its course within the cancellated struc- 
ture of the bone, sending off minute branches which supply the anterior 
teeth, the terminal branches anastomosing with the artery of the opposite 
side. 

The mental branch passes out through the mental foramen accom- 
panied by the mental branches of the inferior dental nerve, and supplies 
the tissues of the chin and lower lip. 

The Veins. — The blood, in returning from the teeth to the heart, 
is first taken up by the posterior dental and inferior dental veins, which 
in their course follow closely that of their corresponding arteries. These 
veins, in conjunction with others which accompany branches of the inter- 
nal maxillary artery, form the pterygoid plexus. At the posterior con- 
fluence of this plexus the returning blood empties into the internal maxil- 
lary vein. Accompanied by the internal maxillary artery it passes back- 
ward and outward, enters the parotid gland, and finally empties into the 
temporomaxillary vein midway between the zygoma and the angle of the 
jaw. After leaving the substance of the parotid gland, the temporo- 
maxillary vein passes downward until near the angle of the jaw, where 
it divides into two branches, one of which passes downward and slightly 
forward, uniting with the facial to form the common facial vein, 
and the other, after passing downward and backward, empties into the 
external jugular vein. The external jugular vein returns the principa 
portion of the blood from the teeth, and from its point of beginning it 
passes almost perpendicularly downward and empties into the sub- 
clavian vein, which, by joining with the internal jugular vein, forms the 
innominate vein, which, in turn, empties into the superior vena cava, 

thus communicating with the heart. 

« 

THE NERVE-SUPPLY TO THE TEETH. 

The nerves supplying the teeth are derived from branches of the 
fifth cranial nerve, otherwise known as the trifacial or trigeminal nerve. 
The fifth nerve is the largest of the cranial nerves, and consists of two parts, 
a large root (sensory) and a small root (motor). The larger portion passes 
into a ganglion (the Gasserian ganglion), frequently compared to the 
ganglion on the posterior root of the spinal nerve. It arises, or makes its 
appearance, at the surface of the brain, on the anterior part of the side 
of the pons Varolii. The sensory root, which, through its branches, 



9 6 



ANATOMY. 



supplies the teeth, is composed of from 80 to 100 filaments, each inclosed 
in a neurilemma, the entire bundle being bound together in a single nerve. 

The fifth nerve is divided into three divisions: First, or ophthalmic; 
second, or superior maxillary; and third, or inferior maxillary (mandibu- 
lar). The branches which supply the teeth are included in the second and 
third divisions, the upper teeth being supplied by branches from the 
superior maxillary nerve, and the lower teeth by branches from the in- 
ferior maxillary nerve. 

The Second Division, or Superior Maxillary Nerve (Fig. 53). 
This nerve, composed entirely of sensory fibers, is intermediate in size 



AXTERWR VEXTAL MAXILLARY XERYE ORBITAL BRANCH 



VAX1LLARY 




AT/; 



ME< 'EEL'S GANGLION 



POSTERIOR DENTAL 



LOOP FORMED BY MIDDLE AXD ANTERIOR DENTAL NERVES 
Fig. 53. — The Maxillary Nerve, seen from Without. {Morris, after Beaunis.) 



between the inferior maxillary and the ophthalmic divisions. It passes 
forward from the Gasserian ganglion and leaves the cranium through the 
foramen rotundum. It traverses the upper part of the sphenomaxillary 
fossa, and passes into the orbit through the sphenomaxillary fissure; then 
passes forward along the infra-orbital groove, and enters the infra- 
orbital canal, where it receives the name of the infra-orbital nerve. Pass- 
ing through this canal, it emerges upon the face through the infra-orbital 
foramen. The superior maxillary nerve, beside supplying the teeth, 
sends off branches to the dura mater, to the orbit, and terminal branches 
in three groups — labial, nasal, and palpebral. The branches given off 



THE NERVE-SUPPLY TO THE TEETH. 



97 



to the teeth are the posterior superior dental, the middle superior dental, 
and the anterior superior dental. 

The posterior superior dental arises from the second division of the 
fifth nerve, by one or two roots, just before it passes into the infra-orbital 
canal. It is divided into a superior and an inferior set; the former passes 
forward and terminates in the canine fossa, while the latter, usually the 
larger, enters the posterior dental canals, and, following the line of the 




Fig. 54. — Dissection showing Mandibular (Third) Division of Fifth Nerve, a, Temporal 
Bone; b. Mental Branches emerging through Mental Foramen; c, Lingual Nerve; d, Man- 
dibular or Inferior Dental Nerve. 



alveolar process through minute canals in the bone, sends off twigs to the 
molar teeth, ending in a plexiform manner by communicating with the 
middle superior dental nerve. This nerve is also distributed to the gums 
and adjacent buccal mucous membrane. 

Middle Superior Denial Nerve. — The infra-orbital nerve, soon after 
entering its canal, gives off this branch, which passes outward, downward, 
and forward over the outer wall of the maxillary sinus, and, after forming 
7 



98 ANATOMY. 

plexuses with the posterior dental branches, gives off filaments to supply 
the bicuspid teeth. 

The anterior superior dental nerve, which is the largest of the dental 
set, is given off from the infra-orbital nerve, enters a canal close to the 
infra-orbital foramen, passes over the anterior wall of the maxillary 
sinus, and, after communicating with the middle and posterior dental 
nerves, divides into ascending and descending branches, the latter being 
distributed to the incisor and cuspid teeth. 

The Third Division, or Inferior Maxillary Nerve (Fig. 54). — 
This is the largest of the three divisions of the fifth nerve, and is both 
motor and sensory in its function. Besides being distributed to the 
lower teeth, it sends filaments to the lower portion of the face, the muscles 
of mastication, the tongue, and mandible. It arises from the Gasserian 
ganglion, passes downward, and emerges from the skull through the fora- 
men ovale, after which it divides into a small anterior (motor) branch 
and a large posterior (sensory) branch. 

The Inferior Denial Nerve. — This is the largest branch of the inferior 
maxillary nerve. From its point of origin it passes downward internally to 
the external pterygoid muscle, and, upon reaching a point between the 
ramus of the mandible and the sphenomandibular ligament, it enters 
the inferior dental canal through the posterior or inferior dental foramen. 
Before entering the foramen, two branches are given off, a lingual and a 
mylohyoid branch. The nerve is accompanied through the inferior 
dental canal by the inferior dental artery, and, when the mental foramen 
is reached, it terminates by dividing into an incisive and a mental branch. 
Between the posterior dental foramen and the mental foramen the nerve 
gives off a series of twigs to the bicuspid and molar teeth, and these, by 
communicating with one another within the substance of the bone, form 
a fine plexus. 

The incisive branch follows the incisive arteries through the sub- 
stance of that part of the bone between the mental foramen and the 
symphysis, and supplies the incisor and bicuspid teeth, while the mental 
branch passes forward to supply the chin and lower lip. 



CHAPTER VII. 

Other Structures Within the Mouth.— The Gums. 
Membrane. — The Alveolodental Membrane. 
Glands, Ducts, Etc. 



-The Mucous 



The Gums (Gingiva). — The gums are formed by a layer of tough 
fibrous vascular tissue, covering the alveoli, closely attached to their 
periosteum, and provided with a free margin (gingival margin), which 



Compact 
Bone 



Epithelium 




Fig. 55. — Section through Gum and Alveolus. 

is closely molded to the necks of the teeth. They are covered on both 
aspects by the general mucous membrane of the mouth, that overlying 
the labial and buccal surfaces being reflected from the lips and cheeks, 

99 



IOO ANATOMY. 

the palatal surface being continuous with that of the hard palate, and 
the lingual surface reflected from the under surface of the tongue and 
floor of the mouth. In the immediate region of the necks of the teeth the 
gums are especially thin and hard, being closely adherent to the peri- 
osteum and alveolodental membrane in this region. In passing toward 
the base of the alveolar walls the tissue becomes less firmly attached to 
the underlying structure, and, when finally passing into the mucous 
membrane of the cheeks and lips, is quite loose and flabby. This condi- 
tion also prevails on the lingual aspect, but the palatal surface remains 
firm throughout, the entire mucous membrane overlying the roof of the 
mouth being similar in structure and attachment to that portion imme- 
diately surrounding the necks of the teeth. In various situations about 
the labial, buccal, and lingual surfaces of the gums small slender folds 
of mucous membrane are found extending to the surrounding tissues. 
These folds, which act as a bridle or curb to the adjacent movable parts, 
are known as the frena of the mouth. The principal frena are found at 
the median line, and are three in number — the frenum labium superioris, 
frenum labium inferioris, and the frenum linguae. The two former 
extend from the inner surface of the lips to the gums, to which their extent 
of attachment is somewhat variable. The frenum labium superioris is 
usually much larger than the frenum labium inferioris, and its attachment 
to the gum frequently extends almost to the gingival border. The frenum 
lingua; extends from the under surface of the tip of the tongue to the lin- 
gual surface of the inferior gums. This is a much stronger fold than 
those connected with the lips. Similar bridles are found in the buccal 
region, usually near the bicuspid teeth, but they are much smaller than 
those at the median line. The gingival margins, or that portion of the 
gums embracing the necks of the teeth, present much variety in outline. 
Instead of encircling the neck of the tooth in a direct line, the margins 
are made up of a series of semicircles. Using the incisive region for 
reference, the labial and palatal margins are concave rootward, while the 
interproximate spaces are partly or completely filled by gum tissue, 
having the outline reversed or convex in the direction of the crowns of 
the teeth. The gingival margin is also termed the "free margin of the 
gum," this name better describing its extent. As previously stated, the 
gums are attached to the periosteum and peridental membrane, but in 
most instances, and particularly before the adult period, the free margins 
of the gums extend beyond the alveolodental membrane, the limit of which 
is formed by the cervical line. That portion of the gum margin beyond 
the cervical line is in close contact with the neck of the tooth, but is not 



MUCOUS MEMBRANE OF THE MOUTH. IOI 

adherent to it, the connecting medium, the alveolodental membrane, not 
being present to form the attachment. The curvature of the gingival 
margins, and the nature of the tissues which enter into their construction, 
are usually considered as strongly indicative of the temperament of the 
individual. Thus, in the bilious temperament the margins are inclined 
to angularity and the tissues rather thick and firm. In the sanguine type 
the outline formed is almost a perfect semicircle, and the tissues are of 
moderate thickness and firmness. In the nervous type the curvature is 
strongly parabolic, and the tissues firm and delicate. In the lymphatic 
the tissues are loose and thick, and the curvature is long and poorly 
defined. In some instances the interproximate spaces are completely 
filled by the gingivae; in others the space is only partly occupied by these 
tissues. The former condition is present when the proximate surfaces of 
the teeth are nearly or quite parallel with each other, thus reducing the 
capacity of the space. The latter condition is present when the crowns 
of the teeth are bell-shaped and the interproximate spaces extensive. The 
labial and buccal surfaces of the gums are more or less broken by numer- 
ous prominences and depressions, all of which accord with the variations 
upon the surface of the bone beneath. 

Some distinction should be made between the gingival line and the 
cervical line, the former referring to the free margin of the gum, and not a 
fixed line, while the latter refers to that positive line established on the 
tooth by the union of the enamel and cementum. 

Mucous Membrane of the Mouth. — The term "membrane" 
in a general sense is one applied to thin layers of tissue, somewhat elastic 
and of a whitish or reddish color. Such tissues are found lining either 
closed cavities or canals which open externally, absorbing or secreting 
fluids, and enveloping various organs. The simple membranes are of 
three varieties, being either mucous, serous, or fibrous. The mucous 
membranes are so called from the clear viscid fluid (mucus) which they 
secrete. They line the various cavities or tracts of the body which com- 
municate with the exterior. The three grand divisions of mucous mem- 
brane are those lining the digestive, respiratory, and genito-urinary pas- 
sages. Lining the entire cavity of the mouth we find the beginning of the 
digestive tract, being continuous with, in many respects similar to, the skin 
on the exterior and performing similar functions within. It is soft, smooth, 
and velvety, of a bright red color, and quite vascular; it is covered on the 
exterior by a layer of epithelial cells overlying the vascular parts. Imme- 
diately beneath this is a network of fibrous connective tissue forming the 
proper mucous membrane, and still deeper is a third layer, somewhat 



102 ANATOMY. 

loose in texture, but composed of fibrous connective tissue, the submucous 
membrane. The oral mucous membrane, at its point of beginning on the 
contiguous surfaces of the lips, is endowed with keen sensibility; it is dry, 
bright red in color, and plentifully supplied with vascular papilla', in 
many of which are sensory nerve terminals. Distributed along the line 
of junction between the integument and the mucous membrane are 
numerous sebaceous follicles, which, however, are devoid of hair-bulbs. 
The characteristic dryness of this surface gradually becomes changed to a 
mucus-secreting one, as that part of the membrane lining the interior of 
the lips is approached. Distributed over the surface of the labial mucous 
membrane are a number of minute openings, the mouths of the labial 
glands, which lie immediately beneath the membrane. The buccal mu- 
cous membrane, or that lining the cheeks, is similar to that covering the 
internal surface of the lips. It is penetrated at various points by the 
mouths of the buccal glands, which, in general, are smaller and less 
numerous than the labial glands. In the region of the second molar 
teeth the membrane is broken by four or five openings of larger size, 
which communicate with the molar glands. The mucous membrane 
covering the hard palate is thick and firm, less brilliant in color than that 
covering the cheeks and lips, and firmly bound down to the periosteum. 
Running from before backward at the median line, the membrane is 
formed into a slight fold, the median raphe, while near the anterior portion 
of the palate are a number of fantastically arranged folds, the ruga (see 
General Description of the Mouth). The thin but rather dense fibrous 
aponeurosis forming the soft palate is covered anteriorly by the oral mu- 
cous membrane. Suspended from the centerof the free margin of the soft 
palate is the uvula, which is likewise covered by mucous membrane, and 
from the base of this, on either side, are two muscular folds, which extend 
outward and downward, forming the anterior and posterior pillars of the 
fauces. The mucous membrane covering the tongue has already been 
described in connection with that organ. From the mouth the digestive 
mucous membrane passes through the fauces, pharynx, and esophagus 
to the stomach, and is so continued throughout the whole digestive 
tract. Other prolongations also pass into the ducts of the salivary 
glands.* 

The Alveolodental or Peridental Membrane. 

This membrane invests the roots of the teeth, and at the same time 
lines the wall of the alveoli. Being reflected from the periosteum cover- 

* For a minute description of the mucous membrane of the mouth see Part ii. 



THE ALVEOLODENTAL PERIOSTEUM OR ROOT MEMBRANE. 



IO3 



ing the outer alveolar walls, it enters the alveolar sockets as a single 
membrane, affording nourishment to the bone on one side and to the 
cementum of the tooth on the other. It is a connective tissue of moderate 
density, and is rich in its nerve- and blood-supply. The general direction 
of its fibers is transverse, being attached at one extremity to the alveolar 
wall and at the other to the 
cementum of the root. The 
connective-tissue fibers are not 
merely attached to the surface 
of the calcified structure, but the 
strength of this attachment is 
greatly increased by the passage 
of the fibers into the substance of 
the bone at one extremity and 
into the lamellae of the cementum 
at the other. In general, the 
membrane is more closely ad- 
herent to the cementum than to 
the bone, usually clinging to the 
former when removed from its 
socket. The nature of the ar- 
ticulation between the tooth-root 
and the alveolar socket, to the 
production of which this mem- 
brane so largely contributes, is 
one peculiar to itself. While 
there is no marked mobility, 
there is, nevertheless, sufficient 
elasticity in the intervening mem- 
brane to provide against the 
severe concussions and lateral 
strains incident to mastication, the former being provided for by the 
general elasticity, while the latter is cared for by specially distributed fibers, 
which serve to return the tooth to its normal position when slightly rotated 
or laterally displaced. This elasticity is greatest in youth and up to the 
meridian of life, after which time it gradually becomes less pronounced. 
The membrane is thickest about the apical ends of the roots and in the 
cervical region, and the distribution of the fibers at these points is some- 
what different from those about the body of the root. In the former 
location they are spread out fan-like from the apex of the root to attach 




Fig. 56. — Root and Membrane of Tooth. 

p,p, Peridental membrane; ap, apical space; 

a, artery; al, al, alveolar process; /, /, dental 
ligament. 



104 ANATOMY. 

themselves to the surrounding alveolar wall, while in the latter they 
pass longitudinally over the alveolar margins to unite with the periosteum 
of the parts. In conjunction with the functions already mentioned, the 
peridental membrane is the median by which all forces applied to the 
tooth-surface are taken up and conveyed to the brain, making it the organ 
of the sense of touch to the tooth. In a normal tooth sensations of pain 
alone are conveyed through the nerves of the pulp. The nerves of the 
membrane act in precisely the same manner as do other sensory-nerve 
terminals, being influenced by the slightest touch applied to the surface 
of the tooth-crown.* In certain conditions of defective hearing the alveo- 
lodental membrane may be made to assist this function by the use of an 
instrument made for the purpose known as a dentiphone. 

Blood-supply to the Alveolodental Membrane. — A very clear 
idea of the blood-supply to this membrane may be obtained from 
figure 56. 

After entering the alveolar socket by one or more arterial branches, 
the thickest portion of the membrane is gained where a number of smaller 
twigs are given off, one or more of which enter the pulp-canal of the tooth- 
root through the apical foramina supplying the pulp, which in turn 
supplies the tooth-structure within, while the others ramify through the 
substance of the alveolodental membrane, through its capillaries, supply- 
ing the cementum from without; while passing through the membrane 
further minute branches are given off which penetrate the walls of the 
alveolus and anastomose with the arteries which supply the oral mucous 
membrane, in this manner providing a generous blood-supply to the parts. 
Further on we shall see that the tooth-pulp and the alveolodental mem- 
brane spring from the same source (see Development of the Teeth), and 
the blood-supply to the parts during the saccular stage of development 
is alike distributed to the base of the pulp and to the follicular walls. 
After the completion of the developmental process this distribution is but 
little changed, the blood-vessels accommodating themselves to the altera- 
tions incident to the generation of the parts. 

The Nerve-supply to the Alveolodental Membrane. — The nerve- 
supply to this membrane is distributed in a manner similar to the blood- 
supply, being derived from filaments given off from the dental nerve and 
entering the tooth-socket by the side of the blood-vessels, and by numer- 
ous filaments which reach the structure by passing through the many 
minute canals in the substance of the alveolar walls and the intervening 
septa. 

*This membrane is more fullv described in Part ii. 



GLANDS OF THE MOUTH. 105 



GLANDS OF THE MOUTH. 



The glands of the mouth are of two kinds, being either serous or 
mucous, and, as they differ in the character of their secretions, so they 
differ in structure. The mucous glands are the most numerous, and are 
found beneath the mucous membrane of the lips, in the same membrane 
lining the cheeks, the hard and soft palate, the tonsils, and at the back of 
the tongue. These glands are quite variable in size, but are all of macro- 
scopic proportions, appearing when examined in this manner as minute 
whitish bodies. The secretions from the glands are poured into the 
mouth through small ducts which pass in various directions through the 
substance of the mucous membrane. Beginning as a single duct for each 
gland, they pass to the submucous tissue, here branching into two or 
more smaller ducts terminating in alveoli, the number and size of these 
depending upon the size of the gland with which they are connected. 
The glands are variously named according to their location, those occupy- 
ing the lips being known as labial glands; those of the cheeks, the buccal 
glands; those of the palate, the palatal glands, etc. The mucous glands, 
although differing in size, are similar when histologically considered. 

The Labial Glands. — These are among the largest mucous glands 
of the mouth, and are more numerous in the upper than in the lower lip. 
The form, size, and location of the labial glands may best be studied 
by dissection, which may readily be accomplished by first removing 
the integument and muscular tissues from the parts, when they will be 
brought into view. The glands are irregularly arranged, and are most 
numerous near the median line. The body of each gland is surrounded and 
held in position by connective tissue, as well as by the duct connecting 
it with the interior of the mouth. 

Besides the mucous glands of the lips, there are present numerous 
sebaceous glands. These are somewhat smaller, and are situated beneath 
the mucous membrane covering the contiguous surfaces of the lips, the 
numerous ducts leading from them opening upon these parts. 

The Buccal Glands. — The glands of the cheek, otherwise known 
as buccal glands, are similar to, but smaller than those of the lips, and are 
placed between the submucous tissue and the buccinator muscle. These 
glands also pour their secretions into the mouth through numerous ducts 
which pass through the buccal mucous membrane. 

In the region of the third molar teeth another set of mucous glands 
open into the mouth, known as the molar glands. They are placed between 
the buccinator and masseter muscles, are similar in construction, and 



106 ANATOMY. 

secrete a like fluid to those previously described, being larger than the 
buccal and smaller than the labial glands. 

Palatal Glands. — Situated between the mucous membrane of the 
hard palate and the periosteum are numerous mucous glands similar to 
those previously described. Provision is made for the accommodation 
of these glands by many small depressions in the bony plates (see Fig. 6). 
They are irregularly distributed over the surface of each lateral half of 
the hard palate, but are absent at the median raphe and immediately 
beneath the ruga?. The mucous membrane of the hard palate is tense 
and hard, in consequence of which it is not so thick as the buccal and 
lingual membranes, and the ducts from the numerous glands are there- 
fore not so long. The glands of the 59ft palate, uvulae, and fauces are 
situated beneath the deep layer of mucous membrane covering these 
parts, in the former structure opening on both the oral and nasal surfaces. 

Lingual Glands. — The glands of the tongue are of two kinds — 
mucous and serous. The former are chiefly found at the back part of the 
tongue, but a few of smaller size are present near the tip. The serous 
variety are to be found only at the back of the organ, and are closely asso- 
ciated with the taste organs in this region. These glands are assisted in 
performing their function by being placed between bundles of striped mus- 
cular tissue, the activity of which forces the secretions to the surface 
by compressing the glands. While the majority of the lingual glands are 
present in the circumvallate region, a number are found distributed 
beneath the mucous membrane of the borders and tip of the organ. 

The Salivary Glands (Fig. 57). — These glands, while outside the 
mouth, are so closely associated with its functions that a brief description 
will be presented. The chief salivary glands are six in number, three on 
each side. They are named parotid, submaxillary, and sublingual; the 
former, secreting true, thin, watery saliva is a true salivary gland, while 
the latter two are known as mixed, or mucosalivary glands, secreting 
both mucus and saliva. 

The Parotid Gland. — This gland is the largest of the three, and is 
placed a little below and in front of the ear, having the following bounda- 
ries: Anteriorly, by the ramus of the mandible; posteriorly, by the styloid 
and mastoid processes of the temporal bone; above, by the root of the 
zygoma, and below, by a line drawn backward from the angle of the jaw. 
While the extent and outline of the gland is somewhat variable, its posi- 
tion is approximately that outlined in figure 57. Its superficial surface, 
somewhat lobulated, is in close relation to the skin and fascia, while 
deeply it penetrates well into the neck by two processes, one of which 



GLANDS OF THE MOUTH. 



107 



passes behind the styloid process and beneath the mastoid process of the 
temporal bone and the sternomastoid muscle, while the other passes in 
front of the styloid process. Given off from the body of the gland and 
extending in various directions are a number of processes or lobes, one 
extending forward between the two pterygoid muscles, and known as the 
pterygoid lobe; another passing into the glenoid cavity, the glenoid lobe; 
while a third passes deeply between the carotid vessels, and is called 



SOCIA TAROTIDIS 



DUCT OF SOCIA 
PAROTIDIS 



DUCT OF PAROTID 



Bristle inserted 
into duet 



Frsenum linguas 

DUCT OF R1VINUS 



SUBLINGUAL GLAND 



PAROTID GLAND 



Masseter muscle 



DUCT OF SUBMAXILLARY 
GLAND 
Mylo-hyoid muscle 



Anterior belly of 
digastric muscle 




Sterno-mastoid 
muscle 

Posterior belly of 
digastric muscle 



SUBMAXILLARY GLAND, 
DRAWN BACKWARDS 



Loop of fascia 



DEEP PORTION OF SUBMAXILLARY GLAND 

Fig. 57. — The Salivary Glands. (Morris.) 

the carotid lobe. In many instances there is an additional lobe, which 
is detached from the body of the gland, known as the socia parotidis. 
When present, this lobe is placed over the parotid duct and empties into it. 
Passing through the substance of the gland are a number of arteries and 
veins, principal among which are the external carotid, transverse facial, 
and internal maxillary, the gland receiving its blood-supply by branches 
from these. The internal carotid artery and internal jugular vein lie 
close to its internal surface. The facial nerve and its branches and the 
great auricular nerve pass through the gland from before backward, and 
supply its substance with nerve-force. The, weight of this gland is from 
one-half to one ounce. 



108 ANATOMY. 

The Parotid Duct. — Leading from the gland to the mouth is the 
parotid or Stenson's duct. After passing through the fat of the cheek 
and the fibers of the buccinator muscle, the duct comes in contact with the 
deep layer of the oral mucous membrane. After passing between this 
structure and the cheek tissues for a short distance it enters the mouth 
opposite the crown of the upper second molar tooth, the orifice of the 
duct appearing on the surface of the mucous membrane in the form of a 
small papilla, which may be readily observed with the naked eye. When 
first given off from the gland a number of small ducts are present, but 
these soon unite and form a single canal. The parotid duct is quite 
dense, of considerable thickness, and is lined by a reflexion of the buccal 
mucous membrane. 

The Submaxillary Gland. — This gland, which receive its name from 
occupying a position below the maxillary bone, is somewhat smaller 
than the parotid. It is situated beneath the mylohyoid ridge, and occu- 
pies the anterior part of the submaxillary triangle, extending upward to 
occupy the submaxillary fossa on the lower border of the maxilla. Super- 
ficially, it is covered by the skin and a few muscular fibers and the deep 
fascia. The facial vein and branches of the facial nerve pass over its 
superficial surface. Deeply, it is in relation with the mylohyoid and 
hyoglossus muscles, and also with the mylohyoid artery and nerve. 
The gland receives its blood- and nerve-supply from the arteries and 
nerves which penetrate it. This gland is separated from the sublingual 
gland by the mylohyoid muscle, and weighs about two drams. 

The Submaxillary Duct. — The submaxillary duct, otherwise 
known as Wharton's duct, passes forward and inward, opening into the 
cavity of the mouth on the summit of a small papilla near the frenum 
lingua?. The duct, which is nearly two inches in length, first passes 
through the adjacent muscular tissue, and finally beneath the oral mucous 
membrane to its outlet. Like the parotid duct, it is lined by a reflexion 
of the oral mucous membrane. 

The Sublingual Gland. — This is the smallest of the salivary glands, 
and is also named from its location beneath the tongue. Its position is 
beneath the tip of the tongue and the mucous membrane covering this 
part of the floor of the mouth. It rests in the submaxillary fossa of the 
maxilla, meeting with its fellow at the median line, and extending as 
far back as the mylohyoid muscle, which separates it from the sub- 
maxillary gland. The gland is supplied with blood from the sublingual 
and submental arteries, and with nerves from the gustatory. The weight 
of this gland is about one dram. 



GLANDS OF THE MOUTH. IO9 

The Sublingual Ducts. — This gland communicates with the 
mouth by one large duct — the duct of Rivini — which springs from the 
main portion of the gland, and by a number of smaller ducts, eight to 
twenty in number, which open on the floor of the mouth. The duct of 
Rivini follows the submaxillary duct, and opens with it at the same papilla. 
The smaller ducts are given off from a number of little lobes which cluster 
about the fore part of the gland. 



CHAPTER VIII. 

A Description of the Upper Teeth in Detail. — Calcification, Erup- 
tion, and Average Measurements. — Their Surfaces, Ridges, 
Fossae, Grooves, Sulci, Etc. 

UPPER CENTRAL INCISOR. 




I St 


3<i 


6th 


7th 


8th 


qth 


10th 


nth 


year 


year 


year 


yea r 


year 

Fig. 58. 


yeai 


year 


year 



Calcification Begins, from Three Centers, First Yeab after Birth. 
I ilcification Completed, Tenth ro Eleventh Year. 
Erupted, Seventh to Eighth Year. 
Average Length of Crown, 59 

Average Length of Root, .4Q. 

\\ erage Length over All, .88. 

During the first year after birth this tooth begins to calcify, this 
process taking place along the future cutting-edge of the tooth in 
three distinct lobes or plates, which afterward unite and form three 
eminences or tubercles, the lines of this union being indicated upon the 
completed crown by two more or less defined grooves — developmental 
grooves. By the end of the third year the deposit of lime-salts has carried 
the process of calcification to a point about midway between the cutting- 
edge and the cervical line. By a continuation of this formative action 
the calcification of the crown is completed between the fifth and sixth 
year. At the beginning of the seventh year calcification has progressed 
to such an extent that the neck of the tooth and base of the root are fully 
outlined. Between the seventh and eighth year the cutting-edge of the 
tooth begins to make its appearance through the gum at a point either to 
the rightfor left ot the median line, and, by a gradual absorption of the 

lie 



UPPER CENTRAL INCISOR. 



Ill 



gum tissue, eruption takes place. During the following year about one- 
eighth of an inch has been added to the length of the root. At the end of 
the eighth year the root has become calcified to about one-half of its 
completed length. During the ninth year, owing to a reduction in the 
diameter of the root, the extent of growth has almost doubled that of 
the previous year, and a decided narrowing of the free root margins is 
to be observed. At the eleventh year calcification is completed in the 
outer root walls, and may be said to be completed (Fig. 58). 

The Crown of the Upper Central Incisor presents for examination 
four surfaces — labial, lingual, mesial, and distal; two angles — a mesial 
and a distal; and a cutting-edge. The 
general form of the crown is that of a 
double inclined plane, or wedge-shape, 
the cutting-edge representing the junc- 
tion of the two sides of the incline, one 
of which looks anteriorly (labial) and 
the other posteriorly (lingual). The 
labial side of the incline is convex, 
while the lingual is concave from the 
cutting-edge toward the root; but, 
upon reaching its upper or cervical 
third, it presents a slight general con- 
vexity. The base of the wedge is 
directed upward and partakes of the 
contour of the neck of the tooth. Fig. 59. 

The Labial Surface of the 
Crown (Fig. 59). — In general outline this surface resembles an imper- 
fect quadrilateral. The margins of the surface are the mesial, the distal, 
the cervical, and the incisive. The mesial margin begins at the lower 
border or cutting-edge and passes upward, usually with a slight distal in- 
clination, gradually uniting with the cervical margin. The distal margin 
begins at the cutting-edge and passes upward with a slight mesial 
inclination, also joining the cervical margin. Both of these margins 
possess more or less general convexity, and, at their junction with the 
cutting-edge, form the mesial and distal angles of the crown. The 
cervical margin is rounded and gradually passes into the two lateral 
margins just described. The incisive margin is marked by the cutting- 
edge, and extends from the mesial angle on one side to the distal 
angle on the other. These four margins, which assist in giving to the 
tooth its typal form, are quite variable. This difference particularly 



Cervical 
Ridge 



Mesia 
Angle 




Cutting Labial 
Edge Grooves 

—Upper Central Incisor, 

Labial Surface. 



112 



ANATOMY. 



marked on the mesial and distal margins, where, in some cases, there 
is a decided convergence in the direction of the root, forming what is 
commonly termed the bell-shaped crown, while in others the same 
margins will be nearly parallel with each other, making the width of the 
crown almost as great at the cervical margin as at the cutting-edge. The 
mesial angle is usually pointed and square, while the distal is much rounded. 
This surface of the crown is slightly convex from above downward, as well 
as from side to side, and in the majority of instances is of greater vertical 
than transverse extent. Beginning at the incisive margin are two slight 

longitudinal depressions or grooves — 
the labial grooves — which are resultant 
from the developmental lobes pre- 
viously composing the primitive cutting- 
edge, and, for this reason, are otherwise 
known as developmental grooves. In 
many instances one or more transverse 
ridges are found upon the cervical por- 
tion, but these are supplemental in 
character. 

The Lingual Surface of the 
Crown (Fig. 60). — This surface has its 
borders formed by three marginal 
ridges and the cutting-edge. The 
marginal ridges are pronounced eleva- 
tions of enamel, and surround the 
surface upon three sides, the intervening space in many instances 
being a decided concavity or fossa — the lingual fossa. The mesial 
marginal ridge begins at the mesial angle of the crown, passes upward, 
inward and backward, following the curvature of the mesial border. The 
distal marginal ridge begins at the distal angle of the crown in a somewhat 
less pronounced form, passes upward, backward, and inward, following 
the curvature of the distal surface. Upon reaching the cervical portion 
of the crown these two margins unite and form the cervicomarginal ridge. 
This ridge may be bold and prominent, or it may be but slightly developed. 
Near its center it is frequently broken by a depression or pit — the lingual 
pit. In some instances this pit is deeply penetrating; in others it assumes 
the form of a fissure, and may completely sever the ridge. This border 
is sometimes elevated into a slightly developed tubercle or cusp — the 
cuspule. When this is present it has the appearance of being produced 
by a fold of enamel, and encircling it is a well-marked fissure. The 



Cervical 
Ridge 
Distal 
Marginal 
Ridge 

Distal 
Angle 




Cervical 
Line 

Mesial 

Marginal 

Ridge 



Mesial 
Angle 



Lingual Fossa 

Fig. 60. — Upper Central Incisor, 
Lingual Surface. 



UPPER CENTRAL INCISOR. 



J1 3 



Ceivica] 
Line 



lingual fossa is usually traversed by two longitudinal grooves, which 
correspond to the developmental grooves of the labial surface. When 
a cuspule is present, the fissure which surrounds it frequently passes 
into the fossa, or the fossa may be partly covered by the cuspule over- 
hanging its cervical portion. When the cervicolingual fissure exists, it is 
not unusual for it to bifurcate and throw a branch along the inner border 
of each marginal ridge, or it may penetrate the fossa proper and divide 
it into two parts. The lingual surface is somewhat less in extent than the 
labial surface, this reduction being principally in a mediodistal direction, 
the length of the two surfaces from the 
cutting-edge to the cervical margin 
being about equal. 

The Mesial Surface of the 
Crown (Fig. 61). — The outline of this 
surface resembles an inverted cone or 
triangle, the lines of which are more 
or less broken, the apex of the cone 
terminating at the cutting-edge and 
the base directed toward the root of 
the tooth. The base of the cone is 
made concave by the enamel margin 
or cervical line. The margins of the 
mesial surface are the labial, the 
lingual, and the cervical. The labial 
margin is convex and rounded through- 
out its entire extent, from the cutting-edge to the cervical line. The contour 
of this margin varies with the typal form of the crown, in some presenting a 
decided and well-marked convexity, in others being but slightly curved. 
The lingual margin is concave and rounded, but the line is much broken. 
Beginning at the cutting-edge, it is decided and square, this feature 
usually including the lower third. As it passes upward and the center 
is approached, the line is more concave and rounded in a mesiolingual 
direction, this latter feature increasing upon approaching the cervical 
line. The cervical margin is that formed by the cervical line. It is 
usually well-defined, being concave or V-shaped, with the point of the V 
more or less rounded, and with its free ends pointing one in a labial and 
one in a lingual direction, the former being a trifle longer than the latter. 
The surface between the borders presents a slight general convexity, 
but with an inclination to flatness near the cervical portion, which is 
occasionally developed into a slight concavity. Whatever deviations 

8 




Ceivico- 
lingual Ridge 



Mesial Angle 



Fig. 61. — Upper Central Incisor, 
Mesial Surface. 



U4 



ANATOMY. 



Lingual 
Fossa 



Lingual 
Ridge 




1- !(,. <)2 



may be present in the borders of this surface, from those assumed in the 

description just given, their union at the cutting-edge will always be in 

a direct line with the long axis of 
the tooth. 

The Distal Surface of the 
Crown (Fig. 62) . — In a general way, 
this surface resembles the mesial 
surface just described. There are, 
however, one or two minor points 
of distinction: the borders are all 
more rounded, the labial border 
presenting a greater convexity, and 
the lingual a more perfectly formed 
concavity. The surface is quite 
full in the center, from which it 
slopes away in all directions, thus 
producing a decided general con- 
vexity. The cervical margin of the 

surface is almost identical with the cervical margin of the mesial surface. 

The distance in a direct line from the cervical border to the cutting-edge 

is a trifle less than the corresponding measure- 
ment on the mesial surface. The distal angle 

is equally constant in its position, and, being 

connected with the mesial angle in a direct 

line by the cutting-edge, finds this latter margin 

always in the labiolingual center of the crown. 
71ie Cutting-edge of the Central Incisor. — 

The cutting or incisive edge receives its name 

from its function, that of cutting or inci>in<^ 

the food. It is formed by the junction of the 

labial and lingual surfaces of the crown, and 

extends almost in a direct line from the mesial 

to the distal surface; at its union with the 

mesial surface it assists in forming the mesial 

angle of the crown, and serves the same pur- 
pose by its union with the distal surface. In 

the majority of instances it is an unbroken Surface showing Develop- 

■> J mental Grooves — a 

line. In passing from the mesial to the distal 

angle it converges slightly in the direction of the root, thus making the 

crown a trifle shorter on the mesial than on the distal side. In the 




Fig. 63. — A Young Upper 
Central Incisor, Labial 



UPPER CENTRAL INCISOR. 115 

recently erupted tooth (Fig. 63) the line is broken by the developmental 
grooves; these usually disappear by wear, but occasionally traces of their 
existence remain, and thus permanently break the positive line that 
would otherwise be present. As the cutting-edge approaches the distal 
angle of the crown it is inclined to slope away, producing a less positive 
angle than the corresponding mesial angle. In some instances the 
cutting-edge is quite thin and inclined to sharpness, in others it is 
blunt and dull, the former condition being present when there is a 
decided overbite in occlusion, the latter occurring when this feature is 
less pronounced. The cutting-edge is frequently referred to as the 
occlusal surface, this term being employed to make the description more 
uniform with the bicuspids and molars, and for this reason is permissi- 
ble; but it is only in rare instances that the surfaces occlude directly with 
the opposing teeth, the condition being most frequent in teeth of the 
lymphatic type, and in cases of malocclusion. 

The Cervical Margin. — This margin, which is distinctly outlined 
by the free extremity of the enamel covering of the crown, also marks 
the extent of the membrane covering the root. The margins formed by 
this line are those of a double concavity and a double convexity. On 
the labial and lingual portions it is concave rootward, while on the 
mesial and distal sides it is convex in this direction. If a line be drawn 
around the tooth at the extreme upper point of the enamel covering, 
it will be found to touch only the labial and lingual prolongations, while 
a space will exist between the line thus drawn and the cervical margins 
of the proximate surfaces. The character of the cervical curvature 
varies with the type of the tooth, being more or less pronounced as the 
case may be. In a typical central incisor the cervical line of the labial 
surface will usually form the segment of a larger circle than that of the 
lingual, and, while the mesial convexity may be gracefully curved, the 
distal may incline to angularity. 

The Neck of the Tooth. — The neck of this tooth partakes of a form 
between that of the crown and root which it joins. It is principally 
formed by a sudden sloping of the enamel margin to meet the root. It 
is broader on the labial than on the lingual surface, and is somewhat 
flattened laterally, with an occasional depression or concavity on its 
mesial portion. The neck of this tooth is seldom a decided anatomic 
feature, being less pronounced than upon any other tooth. In the bicus- 
pids and molars both the crowns and the roots assist in forming the neck 
by a constriction of their adjacent parts, while in this tooth the crown 
alone is instrumental in this direction. 



Il6 ANATOMY. 

The Root of the Upper Central Incisor. — The root of this tooth 
is conic in form, its base directed downward, its apex upward. Viewed 
in transverse sections its outline is that of a rounded triangle, one side 
of which faces in a labial, one in a mesiolingual, and one in a distolingual 
direction. The labial side is the most flattened, while the two remaining 
sides are of equal length and oval in form. This triangular outline 
usually continues throughout the entire length of the root, but in some 
instances, near the apical end, may have a decided or slight distal curve, 
included in which will be a more circular form. The taper of the root 
from the base to the apex is very gradual upon the labial and lingual 
surfaces, until the apical third is reached, when the two sides converge 
more rapidly. The mesial and distal surfaces are somewhat flattened 
and taper very gradually from the base to the apex. In a majority of 
instances the root is much longer than the crown, but in rare cases its 
length is barely equal to, or less than, that of the crown. 

Bilious Type. — The crown of the upper central incisor in this 
type is of greater longitudinal than transverse extent; large in size, abound- 
ing in angles rather than curves. It possesses neither brilliancy nor 
transparency of surface, but is slightly inclined to translucency. The 
labial surface is flat, with more or less decided transverse ridges in the 
cervical portion. The labial grooves are generally present in the form 
of well-defined depressions. On account of the angular nature of the 
tooth, this surface approaches closely to the quadrilateral form. The 
mesial and distal surfaces are flat, with their margins bold and well 
defined. The lingual surface also shows the angular nature of the crown 
in having its marginal ridges squarely set and its developmental grooves 
definitely outlined. The cutting-edge is rather thin, square, and sharp, 
the line frequently being imperfectly formed. The mesial and distal 
angles are both well produced, and the cervical margin, in keeping with 
the rest of the parts, is inclined to angularity. 

Nervous Type. — The central incisor common to this temperament 
is delicate and graceful in outline. The crown is of medium size, with 
the length predominating over breadth. The enamel is inclined to trans- 
parency, and is of a blue or bluish-gray color, presenting much brilliancy. 
The labial surface is fairly well rounded, and the labial grooves are 
present as slightly rounded depressions, which frequently extend well 
toward the cervical margin, where they gradually disappear. In general 
outline this surface partakes of the triangular form, the crown of the 
tooth being broad at the cutting-edge and much constricted at the neck. 
The mesial and distal surfaces show a convexity in every direction, 



UPPER CENTRAL INCISOR. 117 

and the nature of the occlusion is manifest from the decided wedge- 
shape appearance of the crown providing for a long overbite. Upon 
the lingual surface but little in the way of detail is to be observed, the 
entire surface from the cutting-edge to the cervical ridge being smooth 
and concave. The cuspule previously referred to is occasionally present 
in this type, breaking the general smoothness of the surface with its 
prominence. The marginal ridges are poorly defined; the cutting-edge 
is a sharp, unbroken line;- the mesial and distal angles are present in 
the form of long, graceful curves, rather than definite angles, this being 
particularly true of the distal angle. The cervical line is decidedly 
curved, the labial and lingual portions being deeply concave rootward, 
while the mesial and distal are decidedly convex. 

Sanguineous Type. — The crown of the central incisor is usually 
above the average in size, but is well proportioned, abounding in curves 
and rounded outlines. The enamel is inclined to translucency, particu- 
larly near the cutting-edge. The labial surface is smooth and rounded; 
the depressions formed by the labial grooves are slightly observable, 
and extend but a short distance from the cutting-edge. The surface 
is somewhat greater in longitudinal than in transverse extent, and ap- 
proaches much nearer to a circular form than the corresponding tooth 
of other types. The mesial and distal surfaces are well rounded, making 
the point of contact with approximating teeth near the center of the 
surface. The lingual surface abounds in heavy rounded lines; the 
marginal and cervical ridges are particularly prominent, diminishing 
the extent of the lingual fossa. A cuspule is frequently present in the 
form of a well-rounded prominence. The cutting-edge is of moderate 
thickness and slopes away from the center in either direction to assist 
in forming the rounded mesial and distal angles. The cervical curvature 
on the labial and lingual surfaces is an unbroken semicircle, while that 
of the mesial and distal surfaces is less uniform. 

Lymphatic Type. — In the central incisor of this typal form the 
crown is large, but not shapely, and the breadth is equal to, or exceeds, 
the length. The enamel coloring is muddy or brownish-yellow, and the 
surface is lacking in brilliancy. The labial surface is flat and smooth, 
with a faint sign of the labial grooves. The general outline of this 
surface is that of a circular cone, with the cutting-edge for the base, and 
the apex formed on the cervical margin. The mesial and distal aspects 
present a striking contrast to the types previously described, by having 
a labiolingual diameter greater than that represented between the cervical 
line and the cutting-edge. These two surfaces are convex in a labio- 



Il8 ANATOMY. 

lingual direction only, making the point of contact with approximating 
teeth an extended surface rather than a single point. 

The lingual surface is heavy and bulky, frequently to such a degree 
as to produce a general convexity rather than a concavity, as found in 
most typal forms. This surface is frequently broken by one or more 
longitudinal grooves, but is seldom crossed by transverse lines of any 
kind. The cutting-edge is barely deserving of the name. Although 
formed by the free borders of the labial and lingual surfaces, these two 
planes are so far separated at their incisive margins that the space between 
them, instead of being an edge, becomes a more or less broadened surface, 
and one upon which the lower incisors frequently occlude. The line 
thus formed is straight and direct from the mesial to the distal angle 
of the crown, both of which are well produced. The cervical curvature 
is represented by the segment of a much larger circle than that found 
upon teeth of other types, and the neck of the tooth is heavy and bulky, 
showing but little constriction at this point. 

UPPER LATERAL INCISOR. 




jth 


5th 


7th 


9 th 


ioth 


i ith 


year 


year 


year 


year 


year 


year 



Fig. 64. 

Calcification' Begins, from Three Centers, First Year after Birth. 
Calcification Completed, Tenth to Eleventh Year. 
Erupted, Seventh to Eighth Year. 
Average Length of Crown, .34. 

Average Length of Root, .51. 

Average Length over All, .85. 

Like the central incisor, calcification in this tooth begins during 
the first year after birth, the process taking place in the same manner, 
from three centers, along the future cutting-edge, and gradually extend- 
ing in the direction of the root. By the expiration of the third year the 
cutting-edge and the angles of the crown are fully formed; the fourth 
year finds the crown calcified to nearly one-half its completed length; 



UPPER LATERAL INCISOR. 



II 9 



by the fifth year the cervical ridge is reached; while the sixth year usually 
completes the process of coronal calcification. At the close of the seventh 
year the base of the root is fully outlined, and during the following year 
about one-eighth of an inch is added to its length, and still greater progress 
is made during the ninth year, by which time fully three-fourths of the 
root length has become calcified. During the tenth year the apical end 
of the root begins to form by a sudden doubling-over of the free cal- 
cifying margins, and by the eleventh year the surface of the root is com- 
plete (Fig. 64). By the above description it will be observed that at the 
time of eruption the root of this tooth is only calcified to about one-half 
of its completed length, and the same may be said of the central incisor; 
but so much time elapses between the beginning of the eruptive stage 
and the period at which this phenomenon is completed that the calcifica- 
tion of the root is usually finished by the 
time the tooth assumes its permanent 
position in the jaw. 

The crown of the upper lateral 
incisor, like that of the upper central 
incisor, presents for examination four 
surfaces — labial, lingual, mesial, and 
distal — a cervical margin, a cutting-edge, 
and a mesial and distal angle. The 
general contour of the crown closely re- 
sembles that of the upper central incisor, 
except that it measures about one-third 
less from mesial to distal, and is a trifle 
shorter from the cutting-edge to the 
cervical line. As in the central incisor, 
the labial and distal surfaces form a 
double incline plane, and unite below to 

form the cutting-edge. The labial side of the incline is convex, while 
the lingual is concave, but seldom so marked as that found upon the 
central incisor. The base of the wedge, or double incline, formed by 
the cervical margin, is correspondingly smaller than that of the crown of 
the central incisor. 

The Labial Surface of the Crown (Fig. 65). — This surface of the 
crown of the upper lateral incisor is more irregular in outline than the 
corresponding surface of the central incisor. The margins of the surface 
are the mesial, distal, cervical, and incisive. The mesial margin begins 
at the mesial angle and passes upward with a decided distal inclination 



Distal 
Angle 




Cervico- 
labial 
Ridge 



Mesial 
Angle 



Labial Grooves 

Fig. 65. — Upper Lateral Incisor, 
Labial Surface. 



120 



ANATOMY. 



Lingual 

Ridge 

Mesial Mar 

ginal Ridge 



Cervical 
Ridge 

Distal Mar- 
ginal Ridge 



to meet the cervical margin. The distal margin is shorter and decidedly 
more convex than the mesial margin, this variation in outline being still 
more marked when compared with the corresponding margin of the 
central incisor. At the cutting-edge these two margins assist in forming 
the mesial and distal angles of the crown, and by their continuation and 
union above form the cervical margin. The incisive margin is formed 
by the cutting-edge. Like the central incisor, the four margins of this 
surface vary greatly in the different types; this is particularly true of 
the two lateral margins, which at times are found to be in the form of a 

direct line, or even slightly concave, 
while in others they are both decid- 
edly convex. This surface of the 
crown shows a greater general con- 
vexity than the labial surface of the 
central incisor, the cervical portion 
presenting a curve much more de- 
cided than that near the cutting- 
edge. The labial grooves are in all 
respects similar to those described in 
connection with the central incisor, 
and extend from the cutting-edge 
toward the center of the surface, where 
they gradually disappear. Trans- 
verse ridges are occasionally found 
near the cervical portion of the surface. 
The Lingual Surface of the Crown (Fig. 66). — This surface of 
the upper lateral incisor is subject to much variation in form, but presents 
the same points for examination as the corresponding surface of the 
central incisor. These consist of the marginal ridges, which are usually 
more pronounced than those of the central, making the concavity or 
fossa between them small and deep. In some instances the surface will 
be smooth and flat, with an entire absence of ridges or fossa.'. The 
distal marginal ridge is shorter and more bowed than the mesial, and the 
cervical ridge is well marked and proportionately broader and stronger 
than in the central incisor. In some instances the marginal ridges are 
but slightly developed, with their cervical ends broadened and separated 
by a deep fissure, giving the appearance of a terminal fold in the enamel. 
The cervical ridge is frequently broken by a cuspule, which is usually 
more pronounced than when found upon the central incisor. The 
lingual fossa may be present as a smooth, unbroken concavity, or it may 




Lingual Grooves 

Fig. 66. — Upper Lateral Incisor, 
Lingual Surface. 



UPPER LATERAL INCISOR. 



121 




1^ ig. 07. — Upper 
Lateral Incisor, 
Mesial Surface. 



be subdivided by a longitudinal ridge, which often exists to such an 
extent as to force the remaining portions of the fossa well against the 
marginal ridges, where they will be observed as slight depressions rather 
than marked concavities. 

The Mesial Surface of the Crown (Fig. 67). 
— Viewing the crown from this aspect, the outline 
is that of an inverted cone or triangle. The lingual 
margin of the surface is well defined, and the angle 
formed by the union of this surface with the lingual 
surface is moderately acute. The labial margin is 
well rounded, and passes into the labial surface 
without a decided line of demarcation. The sur- 
face on its upper or cervical third is usually flat- 
tened and occasionally concave. At the center, and 
continuing toward the cutting-edge, it is decidedly 
convex in every direction, thus producing a promi- 
nent point of contact with the distal surface of the 
central incisor. 

The Distal Surface of the Crown (Fig. 68). 
— This surface also shows the characteristic wedge-shape of the crown, 
and is principally different from the mesial surface in being convex 
throughout. Near the center it is well rounded and full, providing a 
point of contact for the mesial surface of the cuspid. 
The lingual margin, while being more decidedly out- 
lined than the labial, is much more rounded than 
the lingual margin of the mesial surface. From the 
most prominent point near its center the surface slopes 
away in every direction, the convexity being most 
marked near the cutting-edge. 

The Cutting-edge of the Lateral Incisor. — In the 
young tooth the cutting-edge presents the three little 
tubercles common to all incisors, the grooves which 
divide them passing up over the labial and lingual 
surfaces and forming the labial and lingual grooves. 
These soon disappear by wear, usually leaving the 
cutting-edge in the form of a direct line and connecting 
the two angles of the crown. Like the central incisor, 
this margin of the crown may be thin and sharp, or it may be thick and 
dull. 

The Cervical Margin. — The line of demarcation between the crown 




Fig. 68. — Upper 
Lateral Incisor, 
Distal Surface. 



122 ANATOMY. 

and the root of the tooth resembles so closely that described in connection 
with the central incisor that it will only be necessary to mention one or 
two characteristic differences. The lingual side of the line presents a 
much smaller curve proportionately, and usually extends a little higher 
in the direction of the root than that represented upon the labial portion. 
The mesial and distal portions of the line dip well down, decreasing the 
length of the crown on these surfaces; the margin on the former surface 
is usually angular and V-shaped, while on the latter it is circular in form. 

The Angles of the Crown. — The angles of the crown are the mesial 
and the distal, and are formed in the same manner as the same angles of 
the central incisor. The mesial angle is generally well produced, in most 
instances being slightly acute; but when the cutting-edge is thin and 
frail, the angle is frequently much obliterated by wear. That portion 
of the crown of the upper lateral incisor which is usually referred to as 
the distal angle is scarcely worthy of the name. It is usually present as 
a long curve, which begins near the center of the cutting-edge and extends 
well up on the distal surface. This characteristic outline is sometimes 
so pronounced as to completely destroy the cutting-edge, the distal 
surface being carried forward by a long curve ending in the mesial angle. 

The Neck of the Tooth. In this tooth the neck is usually marked 
by a constriction much more pronounced than that found in the central 
incisor. On the labial and lingual surfaces it is principally formed by 
a sudden sloping of the enamel surface rootward, but on the two lateral 
surfaces it is formed by a flattening or slight concavity of both the crown 
and the root. 

The Root of the Upper Lateral Incisor. — The root of this tooth 
is conic in form, and is much more flattened from mesial to distal than 
the root of the central incisor. At its junction with the crown it is circu- 
lar in form, the labial portion forming the segment of a larger circle than 
the lingual, this feature being observed throughout its entire length. The 
flattening of the mesial and distal sides begins immediately above the 
neck, and gradually increases as the center of the root-length is approached, 
where it often develops into a slight longitudinal depression. As the 
apex of the root is approached, this longitudinal depression gradually 
disappears, and the root again becomes circular in form. The thickness 
of the root is about one-third greater from labial to lingual than from 
mesial to distal, and, while it is generally classed as a straight root, it is 
frequently provided with a pronounced distal curve near the apical 
extremity. In some instances it is found with a double mesiodistal 
curve. 



UPPER LATERAL INCISOR. 1 23 

Bilious Type. — In this type the lateral incisor is frequently poorly 
developed, the cutting-edge and distal angle are wanting, the crown 
being in the form of a single conic cusp, the distal surface meeting the 
mesial at a point near the mesial angle. This form of crown might 
be classed as one of malformation, but the fact that it most frequently 
occurs in this temperament would appear to indicate a normal condition. 
When the crown takes the form common to incisors, it is of greater 
longitudinal than transverse extent, the angles are well produced, and 
the mesial and distal surfaces are flat and almost parallel with each other. 
The labial surface is flat and is frequently broken by transverse ridges 
near the cervical portion. The lingual surface presents well-marked 
outlines and margins, a cuspule is seldom present, and the lingual fossa 
is well marked, but not deep. The cutting-edge is thin and sharp, to 
provide for the overbite, which is rather long. The cervical border is 
square and angular. 

Nervous Type. — In this typal form the neck of the tooth is a pro- 
nounced feature. The crown is long and narrow, the constriction 
forming the neck beginning well down on the crown and extending over 
the cervical line to the surface of the root. The labial surface is convex 
in every direction and the labial grooves fairly well defined. The mesial 
surface is convex near the center and cutting-edge, but often shows a 
slight concavity on its cervical portion. The distal surface is rounded 
and smooth. The lingual surface presents a general concavity, a cuspule 
being more frequently present than in other types. The lingual fossa 
is deep, and often extends beneath the cervicomarginal ridge in the form 
of a circular fissure. The cutting-edge is thin and sharp, providing for 
a long overbite; the mesial angle is pointed and well formed, while the 
distal is usually much rounded. The cervical line is well arched, forming 
the segment of a much smaller circle than that seen on the same tooth 
of other temperaments. 

Sanguineous Type. — The crown is well proportioned, with the 
length slightly predominating over breadth, all the surfaces being more 
or less rounded and smooth, showing the crown to be made up of curves 
rather than angles. The labial surface presents a graceful convexity 
throughout; the mesial and distal surfaces are both convex, with their 
margins poorly defined. The lingual surface shows the rounded nature 
of the crown in having its fossa and marginal ridges oval and blending 
one into the other. The cutting-edge is moderately heavy and dull, 
in keeping with the overbite, which is short. The cervical line is made 
up of curves rather than angles. 



124 ANATOMY. 

Lymphatic Type. — In this type the crown is generally of greater 
transverse than longitudinal extent. The neck is poorly produced, the 
crown and root uniting without any marked constriction of the parts. 
The labial surface is much flattened from mesial to distal, and but slightly 
convex in the direction of the long axis of the tooth. The mesial and 
distal surfaces are but little rounded and are nearly parallel with each 
other, so that the contact with adjoining teeth becomes an extent of 
surface rather than a single point. The lingual surface is convex above, 
but as the cutting-edge is approached it becomes flat, but seldom con- 
cave. The marginal ridges are not well shown and the lingual fossa is 
but a slight depression. The angles of the crown are well produced and 
the cutting-edge thick and blunt, this marginal surface frequently occlud- 
ing directly upon the opposing lower teeth. The curvature of the cer- 
vical line is that of a long circle. 



.-> 



UPPER CUSPID. 




5th 


6th 


7th 


8th 


9th 


10th 


nth 


year 


year 


year 


year 
Fig. 69. 


year 


year 


year 



Calcification' Begins, from Three Centers, Third Year after Birth. 
Calcification Completed, Twelfth to Thirteenth Year. 
Krupted, Twelfth to Thirteenth Year. 
Average Length of Crown, .37. 

Average Length of Root, .68. 

Average Length over All, 1.05. 

About the third year after birth calcification begins in the central 
lobe, which is gradually extended laterally, until, at the fourth year 
it is met by the two lateral lobes, which are somewhat later in beginning, 
and by the fifth year the three are united, the former eventually establish- 
ing the single cusp of the tooth and the latter two the mesial and distal 
angles. About the sixth year two-thirds of the crown is formed, and by 
the seventh year the constriction which marks the beginning of the neck 



UPPER CUSPID. 



12: 



of the tooth commences to make its appearance. Between the seventh 
and eighth year calcification in the crown is completed and the cervical 
line established, during the following year nearly one-quarter of an inch 
is added to the length of the root, and by the beginning of the tenth year 
the root is formed for fully two-thirds of its entire length. Between the 
twelfth and thirteenth years or at the time of eruption, calcification is 
completed in the root so far as its surface is concerned (Fig. 69). In 
this latter particular the cuspid tooth differs from most of the others in 
being completely calcified previous to, or about the time of, its eruption, 
the eruptive process in this tooth, therefore, differs from that of the other 
teeth, being fully calcified about the time it makes its appearance 
through the gum tissue. To reach its final position in the arch the tooth 
moves bodily downward, the bone filling in behind; while in the incisors, 
bicuspids, and molars the free calcifying root-extremities remain nearly 
stationary, the crowns being 
forced downward as the lime 
salts are deposited. 

The crown of the upper 
cuspid presents for examination 
four surfaces — labial, lingual, 
mesial, and distal — two margins 
— the cervical margin and the 
cutting-edge — and a mesial and 
a distal angle. In general out- 
line it is of the simplest form, 
resembling the primitive cone- 
shaped teeth of many fishes. 
When viewed by looking directly 
upon the mesial or distal surface, 
the wedge-shape common to the 
incisors is observed. The base 
of the double incline is, however, much broader proportionately than the 
corresponding measurement of the incisors. Looking at the crown from 
a labial or lingual direction, its function, as both a penetrating and 
incising organ, may be observed in the single cusp from which it derives 
its name. The cusp, which is formed at the expense of the cutting- 
edge, divides this latter margin into two distinct portions — the mesial 
cutting-edge and the distal cutting-edge. 

The Labial Surface of the Crown (Fig. 70). — The contour of 
this surface is that of a broken circle more or less perfectly drawn. It 



Cervica* 
Ridge 



Mesial 
Angle 

Mesial Cut- 
ting-edge 




'ervical 
Line 



Distal 
Angle 

Oistal Cut- 
ting-edge 



Labial Ridge 
Fig. 70. — Upper Cuspid, Labial Surface. 



126 



ANATOMY. 



Cervical 
Ridge 



is bounded by five margins — mesial, distal, cervical, mesial-incisive, and 
distal-incisive. The mesial margin is rounded from labial to mesial, and 
slightly convex from the cutting-edge to the cervical line. The distal 
margin is also rounded from labial to distal, presents a greater convexity, 
and is somewhat shorter from the cutting-edge to the cervical line than 
the mesial margin. By a continuation and final union of these two 
lateral margins the cervical margin of the surface is formed, while by 
their union with the cutting-edges the mesial and distal angles of the 
crown are established. The mesial-incisive margin is usually slightly 

concave near its center, although in 
some instances it is convex. The 
distal incisive margin responds to the 
same description, although the con- 
cavity, when present, is nearest the 
point of the cusp. From the summit 
of the cusp these two margins slope 
away to join the mesial and distal 
angles, the distal incline being about 
one-fourth longer than the mesial. 
This surface is generally of greater 

ginalRidg.B I Meao- 

■ marginal longitudinal than transverse extent, 

Ridge 

its greatest mesiodistal diameter being 
from angle to angle, or at a point 
immediately above them. The sur- 
face is convex in every direction, and 
is marked by a central longitudinal 
ridge, usually well defined — the labial 
ridge. Beginning at the summit of the cusp, this ridge is more or less 
contracted laterally, but as it passes over the surface in the direction of 
the root it becomes broadened and flattened, and gradually disappears 
in the cervical portion. Upon either side of this ridge are the labial 
grooves, well defined at their beginning, but which gradually blend into 
the surface of the crown as they pass rootward. In some instances these 
grooves are so strongly defined as to form a decided ridge upon the mesial 
and distal margins of the surface; — these are the labial marginal ridges. 
The labial ridge and the two labial grooves mark the developmental 
lines of the crown, the former resulting from the middle lobe, which 
in this tooth is much the largest of the three, while the latter denotes 
the line of junction between the middle and the lateral lobes. 

The Lingual Surface of the Crown (Fig. 71).— This surface 



Lidgual 
Groove 




Lingual 
Groove 



Fig 



7i-- 



Lingual Ridge 
Upper Cuspid, Lingual Surface. 



UPPER CUSPID. 



127 



presents nearly the same general outlines as the labial, with the exception 
of the cervical portion, which is more constricted, tending to produce an 
oblong or egg-shape. It usually abounds in well-defined ridges and 
depressions, giving to the tooth a rugged and strong appearance. There 
is but little general concavity to the surface in passing from the point of 
the cusp to the root, while it may be flat, concave, or convex. As in the 
incisors, the margins of this surface are formed by three marginal ridges 
and by the cutting-edge. The mesiomarginal ridge is commonly a well- 
defined fold of enamel, beginning at the mesial angle and passing upward 
in the direction of the root, where it 
unites with the cervicomarginal ridge. 
It is sometimes quite narrow and rather 
sharply outlined; at others, it extends 
well toward the center of the surface in 
the form of a well-rounded fold. The 
distomarginal ridge, which is some- 
what shorter than the mesial, begins at 
the distal angle and passes rootward to Ceovical 
meet the cervical ridge. It is well Rid 8 e 
rounded in every direction, but seldom 
so well produced as the mesial. The 
cervicomarginal ridge, which is formed 
by a continuation or union of the two 
former, nearly always partakes of their 
nature, except when broken by the 
presence of a cuspule, which is fre- 
quently found upon this tooth (Fig. 72). 

This small cusp of enamel may be bounded on one or both sides by a 
fissure, which often extends well under the cervicomarginal ridge, and 
sometimes completely separates it from the two lateral ridges. Passing 
through the center of the surface from the summit of the cusp to the base 
of the cervicomarginal ridge is the lingual ridge, which corresponds to 
the labial ridge of the labial surface. This ridge is usually well pro- 
duced at or near the point of the cusp, and may continue so throughout, 
but most frequently becomes reduced in size near the center of the sur- 
face. Between this ridge and the mesio- and disto-marginal ridges are 
two longitudinal depressions — the lingual grooves. 

Mesial Surface of the Crown (Fig. 72). — In general outline this 
surface resembles that of the central incisor, excepting that the wedge- 
shape which it describes is more heavily set and blunt, with the surface 



Labial 
Groovt 




Cervico- 
lingual 
Ridge 



Lingual 
Ridge 



Fig. 72.- 



-Right Upper Cuspid, Mesial 
Surface. 



128 



ANATOMY. 



extending beyond the base of the cone, in the direction of the root, to 
the extent of about one-third of its entire length. In some cases the base 
of "the cone will be on a line with the cervical margin. The lower two- 
thirds of the surface, or that nearest the mesial angle, is convex in every 
direction; this convexity gradually disappears as the center is approached, 
beyond which point it is much flattened, usually ending in a slight con- 
cavity at the cervical margin. When looking directly upon this surface, 
its margins will be found within the profile lines, these being represented 
by the labial ridge anteriorly', and by the lingual and cervical ridges 

posteriorly. The margins, 
three in number, are the 
labial, which is well rounded 
and poorly defined; the //';/ 
gual, more or less distinctly 
outlined and somewhat irreg- 
ular; and the cervical, which 
is represented by the extent of 
the enamel covering of the 
crown; this latter margin be- 
ing concave in the direction of 
the root. The most promi- 
nent point of this surface 
serves as a point of contact 
for the distal surface of the 
lateral incisor, the extent of 
contact being much influenced 
by the type of tooth, but in 
the cuspid this is usually a 
single point rather than an extent of surface. 

The Distal Surface of the Crown (Fig. 73). — This surface in 
many respects is similar to the mesial, particularly in its general outline. 
The extent of surface is somewhat less and the convexity much more 
marked than that of the mesial surface. The position of the distal 
angle, which is the lower boundary of the surface, being much nearer 
the cervical line, makes this surface about one-third shorter than the 
mesial surface. The lateral margins of the surface, which are also within 
the profile lines, differ from those of the mesial in being more clearly 
defined. The cervical margin differs from that of the mesial surface 
by having a concavity with much less depth. As stated above, the surface 
is decidedly more convex than the mesial, the point of contact for the 



Labial Sur- 
face- 



Summit of 
Cusp. 




Cervicolin- 
gual Ridge 



Distal 
.Angle 



Fig. 73.— Left Upper Cuspid, Distal 

Surface. 



UPPER CUSPID. 



129 



Cuspule 
Lingual 
Fossa 



mesial surface of the first bicuspid being almost in the center. Near 
the cervical margin the surface is inclined to flatness, and frequently 
concave. 

The Cutting-edge, or Cusp. — As inferred in the beginning of this 
description, the cuspid tooth is both an incising and a penetrating organ, 
the latter function being provided for by the presence of the single cusp, 
which divides the cutting-edge into an anterior or mesial portion and a 
posterior or distal portion. The mesial cutting-edge begins at the summit 
of the cusp and slopes away to meet the mesial angle, which it assists 
in forming. The outline of this edge is 
usually gracefully curved and unbroken 
unless permanently crossed by the labial 
groove. The distal cutting-edge is gen- 
erally somewhat longer than the mesial. 
Immediately after leaving the summit of 
the cusp it may be slightly concave; but 
beyond this point it is well rounded, 
until it reaches the distal angle, 
which it gradually disappears. This 
edge is also frequently broken by the 
labial groove. In its entirety the cut- 
ting-edge is subject to the same varia- 
tions as those of the incisors — i. e., it may 
be thin and sharp, or it may be thick 
and blunt. 

The Cusp. — The single cusp from which this tooth derives its name 
is formed by the union of the labial ridge, the lingual ridge, and the 
mesial and distal cutting-edges. The summit of the cusp is constant 
in its position, always being in a direct line with the long axis of the 
tooth, whether it be viewed from a mesial or a lingual direction. 

The Cervical Line. — To describe this fully would be to repeat what 
has already been said in connection with the incisor teeth. This enamel 
margin differs in one particular only from that of the incisors, and that 
variation is not a constant one — the lingual portion is frequently extended 
in the direction of the root, producing a short, positive curve at that 
point. 

The Angles of the Crown. — Owing to the rounded, nature of the 
majority of cuspid crowns, the term angle, as applied to its free extremities, 
is almost a misnomer, and can only be considered as assisting in descrip- 
tion. The mesial angle, which is formed by the union of the marginal 

9 



into Cervica 
Ridgt 



Lingual 
Groovt 




Marginal Ridges 

Fig. 74. — Upper Cuspid, Lingual 

Surface, Strongly Developed. 



!3° 



ANATOMY. 



ridges of the labial and lingual surfaces with the mesial cutting-edge, is 
seldom a well-produced angle, usually being rounded in every direction. 
The distal angle, which is formed in a manner similar to the mesial, rs 
somewhat more deserving of the name, both the labio- and linguo-margi- 
nal ridges frequently presenting angularity. The position of the distal 
angle is usually well toward the center of the crown, and occasionally 
above this point, and, although it may descend, it is seldom found on 
a line with the mesial angle. 

The Neck of the Upper Cuspid. — This may or may not be a dis- 
tinctive feature of the cuspid tooth, although when viewed from a labial 
aspect, the lateral flare or bulging of the crown gives the appearance of a 
decided constriction between the crown and the root; but, when examined 
from the mesial surface, this constricted appearance is absent, the con- 
tour of the crown passing into that of the root, with the cervical line alone 
marking the extent of each. The tooth at this point is well rounded 

anteriorly, flattened laterally, and again rounded 
posteriorly, the latter forming the segment of a 
-mailer circle than that of the labial surface. 

The Root. — This tooth possesses the largest 
and longest root of any of the teeth, in the latter 
respect usually exceeding the central incisor by 
about one-third, and the lateral incisor by one- 
fourth or more. Like the base of the crown, it is 
rounded on the labial and lingual surfaces, and is 
flattened laterally, this form usually being continued 
throughout its entire length. Itgradually diminishes 
in size from the neck to the apex, and in its entirety 
forms a perfect cone. On the mesial and distal 
sides it is not only much flattened, but is frequently 
provided with a longitudinal depression, which is 
most marked near the center of its length. In 
some instances this root is possessed of a slight distal 
curve, which may be gradual from the base to the apex, or it may exist 
in a more positive way by a sudden distal curve near its apical extremity. 
Bilious Type (Fig. 75). — The rounded outlines common to the 
cuspid tooth are less pronounced in this type than in any other, and 
instead of curves, angles are present. The crown is above the average 
size, length predominating over breadth, the cusp well formed, and the 
angles strong. The labial surface is often crossed by a number of trans- 
verse ridges near the cervical portion, the labial ridge is bold, as are also 




Fig. 75. — Bilious Type. 
Distal Surface. 



UPPER CUSPID. 



131 



the labiomarginal ridges. The mesial and distal surfaces possess no 
distinguishing features, but the lingual, like the labial, shows the angular 
nature of the crown in having its margins and ridges squarely set. 

A cuspule is more frequently found in this type than any other, 
and sometimes reaches down to a point corresponding to the transverse 
center of the crown. The neck is moderately well produced, and the 
cervical line decidedly V-shaped on its lateral portions, while on the labial 
and lingual it takes the form of a broken circle. The cutting-edges are 
rather heavy and square, and are nearly of equal 
length. In this temperament the cuspid tooth 
often partakes of the form described in connection 
with the lateral incisor of the same type — i.e., the 
absence of the cutting-edge and one or both angles, 
making the crown a perfect cone. 

Nervous Type (Fig. 76). — The crown is of 
much greater longitudinal than transverse extent, 
the outlines oval and gracefully formed, and the 
neck is much constricted from mesial to distal, 
being made so by the lateral flare of the body of 
the crown, which is a distinctive feature of this 
type. The labial ridge is well formed near the 
summit of the cusp, but usually disappears near 
the center of the surface. The labiomarginal 
ridges are seldom present, and the surface in 
general is convex and smooth. The mesial and distal surfaces show a 
pronounced convexity near the angles, and often a slight concavity 
between this point and the cervical line. The lingual surface, while 
generally showing all the descriptive lines, may be considered smooth; 
it is convex from mesial to distal, and slightly concave in the direction 
of the long axis of the tooth. The cusp is long and penetrating, the 
distal cutting-edge is much longer than the mesial, and both are 
inclined to sharpness. The cervical line on the labial and lingual 
surfaces is deeply arched, frequently giving to the gingival margin a 
receded appearance. 

Sanguineous Type (Fig. 77). — The crown in this type abounds in 
long curves, the longitudinal and transverse extents are nearly equal, 
the angles, owing to their circular form, are barely deserving the name, 
while the cusp and cutting-edges are outlined by one long, oval sweep. 
The labial surface is prominent and convex, and the developmental 
grooves are fairly well shown. The mesial and distal surfaces show a 




Fig. 76. — Nervous 
Type, Labial Surface. 



132 



ANATOMY. 



moderate general convexity, while the lingual abounds in well-rounded 
ridges and borders. The constriction forming the neck of the tooth 
is moderate. The cervical line is in the form of perfectly arched curves, 

forming on the labial surface the segment of 
a circle corresponding to the circumference of 
the crown of the tooth. 

Lymphatic Type. — In this type the 
crown is usually greater in its transverse than 
in its longitudinal measurement; it is lacking 
in graceful outline, and may best be described 
as being short, thick, and heavy set. None 
of the surfaces abound in descriptive lines, 
although transverse ridges are sometimes 
present on the labial surface near the cervix. 
Both the labial and lingual surfaces are con- 
vex in every direction, while the mesial and 
distal are inclined to flatness. The cusp is 
heavy and blunt, and the cutting-edges, which 
are nearly of equal length, are thick and 
dull. The mesial and distal angles are well 
produced. The cervical line is almost a 
direct line encircling the neck of the tooth, 
the segmental form on the labial surface 




Fig. 



-Sanguineous 



type, Distal Surfac e. 



being that of a much larger circle than any of those previously described. 
The neck is thick and heavy, and the roots generally short. 



UPPER FIRST BICUSPID. 133 



UPPER FIRST BICUSPID. 




7th year 8th year 9th year 10th year nth year 12th year 

FIG. 78. 

Calcification Begins, from Four Centers, about the Fourth Year. 
Calcification Completed, Eleventh to Twelfth Year. 
Erupted, Tenth to Eleventh Year. 
Average Length of Crown, .32. 

Average Length of Root, .48. 

Average Length over All, .80. 

This tooth, although presenting a crown of vastly different contour, 
is developed by a process almost identical with that of the incisors and 
cuspids. As the name implies, it is made up of two cusps, one forming 
the buccal and the other the lingual half of the crown. Calcification 
in the buccal cusp is from three centers and begins about the fourth year, 
the central lobe first receiving the lime salts. During the following 
year the two lateral lobes begin to calcify, soon followed by a union of 
the three, thus completing the margins and summit of the cusp. Unlike 
the incisors, but corresponding to the cuspid, the middle lobe is much the 
largest of the three, frequently forcing the developmental (buccal) grooves 
well toward the angles of the crown. The development of the lingual 
cusp corresponds to the development of the cervical ridge on the incisors 
and cuspids, except that it has a separate center of calcification, and a 
cusp almost as large as the buccal results. Between the fifth and sixth 
year union between the two cusps takes place, the line of confluence 
being permanently recorded by a well-defined groove, which traverses 
the crown from mesial to distal. This groove, although forced to occupy 
a different position, corresponds to the lingual groove of the incisors 
and cuspids. After the union of the cusps the process of calcification 
is continued into the body of the crown, and by the seventh year it is 



134 



ANATOMY. 



Buccal Triangular Ridge 



Mesial 
Angle 

Central 
Groove 




Buccal 

Cusp 

Buccal 

Groove 

Distal 

Groove 

Distomar- 
ginal 
Ridge 



Lingual Cusp 



more than half completed. The eighth year usually finds the crown 
fully formed and the base of the root or roots outlined. As this tooth 
is generally provided with two roots, the first indication of bifurcation 
will be observed between the eighth and ninth year by a filling-in near 
the center of the mesial and distal walls, which finally become united 
by a thin septum of dentine or cementum. After this period the roots 
calcify separately, and by the middle of the ninth year about one-third 
of their length is established. During the following year, or at about 
the time of eruption, the development of the roots has extended to about 
three-fourths of their complete length, and between the eleventh and 

twelfth year, or at a time corres- 
ponding to that of the crown assum- 
ing its final position in the arch, calci- 
fication is completed (Fig. 78). 

The Crown of the Upper First 
Bicuspid presents for examination 
five surfaces — buccal, lingual, mesial, 
distal, and occlusal. In general, the 
contour of the crown is irregularly 
quadrilateral, being about one-third 
greater in its buccolingual measure- 
It is somewhat flattened from mesial 
to distal, but rounded on its buccal and lingual surfaces. 

The Occlusal Surface of the Crown (Fig. 79). — The contour 
of the crown is best observed by a view of this surface, which may be 
described as trapezoidal or irregularly quadrilateral in form. The 
four margins of the surface are those which represent the four lateral 
surfaces — the buccal, lingual, mesial, and distal, — the latter two being 
in the form of well-defined ridges — the mesio- and disto-marginal ridges. 
The buccal margin is formed by the mesial and distal inclines of the 
buccal cusp; it has a slight buccal convexity, and at its union with the 
proximate surfaces assists in forming the mesial and distal angles of the 
crown. The distal half of this margin is usually somewhat longer than 
the mesial, and the distal angle is less pronounced than the mesial. The 
lingual margin presents a much greater convexity than the buccal, but 
the curve formed is the segment of a much smaller circle. As in the 
buccal margin, the distal half of this margin is the longest, but unlike 
the former, its free extremities pass into the mesial and distal margins 
without producing angles. The mesio- and disto-marginal ridges are 
strong folds of enamel which arise from the mesial and distal angles and 



Fig. 79. — Upper First Bicuspid, 
Occlusal Surface. 

ment than from mesial to distal. 



UPPER FIRST BICUSPID. 1 



3 



converge slightly as they pass to the lingual, where they are gradually 
lost in the lingual margin. 

The Cusps (Fig. 79). — These are two in number, and are named, 
in accordance with their location, buccal and lingual. The buccal cusp 
is the larger and longer of the two. From the summit of this cusp four 
ridges descend — one in a mesial direction, forming the mesial cutting- 
edge of the crown; one in a distal direction, forming the distal cutting- 
edge; one to the buccal surface, the buccal ridge; and a fourth, the buccal 
triangular ridge, descends the central incline. The mesial and distal 
ridges enter into the formation of the mesial and distal angles at their 
extremities; the latter is slightly longer than the former, and both are 
frequently broken near the center by the grooves of development — the 
buccal grooves. The buccal ridge may be well developed and extend 
almost to the cervical line, or it may be slight and disappear near the 
center of the surface. The buccal triangular ridge usually ends some- 
what abruptly in the central groove, but in some instances it is con- 
tinued and joins a similar ridge from the lingual cusp, this union form- 
ing the transverse ridge. The triangular ridge often bifurcates near 
the center of its incline, and is continued in two distinct but smaller 
ridges. The lingual cusp is much less angular than the buccal; the apex 
is usually rounded, while the descending ridges are generally three in 
number instead of four. The mesial and distal ridges are nearly of the 
same length, and pass without interruption into the mesio- and disto- 
marginal ridges. The triangular ridge is less clearly defined than its 
fellow of the buccal cusp, and it is not unusual for it to be entirely 
wanting. The lingual aspect of the cusp is smooth and rounded, 
presenting nothing in the form of a ridge in correspondence with the 
buccal ridge of the buccal cusp. Like the incisors and cuspids, the 
summits of these cusps are usually in a direct line with the long axis 
of the tooth. 

The developmental grooves, all of which are observed upon the 
occlusal surface, are the central, mesial, distal, two triangular, and two 
buccal. The central groove is the most marked, is deeply sulcate, and 
extends through the center of the surface from mesial to distal, ending 
just within the two marginal ridges in two irregularly formed depressions 
or pits — the mesial and distal pits. This groove marks the line of union 
between the buccal and lingual lobes. The mesial and distal grooves 
are not always well defined, but may usually be observed as fine lines 
passing over the central portion of the mesio- and disto-marginal ridges. 
The mesio- and disto-triangular grooves begin in the mesial and distal 



136 



ANATOMY. 



Buccal 
Ridge 

Mesial 
Angle 




Distal 
Angle 



Distal Cut- 
ting-edge 



Buccal Grooves 

Fig. 80. — Upper First Bicuspid, 
Buccal Surface. 



pits, and pass in the direction of the mesial and distal angles, where they 
are either lost, or may be traced as slight depressions passing over the 
buccal ridges near the angles, and they may further continue over the 

buccal surface in the direction of the 
root. These two grooves, together 
with the mesial and distal above re- 
ferred to, form the outlines of the 
mesiobuccal and distobuccal develop- 
mental lobes. The buccal grooves 
will be described in connection with 
the buccal surface. Supplemental 
grooves are seldom found in connec- 
tion with the buccal half of the oc- 
clusal surface, but are occasionally 
present on the central incline of the 
lingual cusp. 

The Buccal Surface of the 
Crown (Fig. 80). — In many respects 
this surface resembles the corresponding or labial surface of the cuspid 
tooth. It is bounded by four margins — occlusal, mesial, distal, and 
cervical. The occlusal half of the surface is formed of the buccal cusp, 
and is cone-shaped, while the cervical 
half is irregularly quadrilateral in form. 
The extent of surface from the cervical 
line to the point of the cusp is usually 
about one-third greater than the greatest 
mesiodistal diameter, which is repre- 
sented by a line drawn from the mesial 
to the distal angle. The form shown is 
that of a general convexity, the summit 
of which is surmounted by a longitudinal 
ridge — the buccal ridge. This ridge, 
which is formed from the central de- 
velopmental lobe, is most pronounced 
near the occlusal margin, and gradually 
disappears near the center of the surface. 
Upon either side of the buccal ridge are 
two grooves — the buccal grooves — which denote the line of union between 
the central and the two lateral lobes, and beyond these are the angles of 
the crown. The buccal ridge springs from the buccal cusp, the summit of 




Lingual 
Groove 



Summit of Lingual Cusp 

Fig. 81. — Upper First Bicuspid, 

Lingual Surface. 



UPPER FIRST BICUSPID. 



1 31 



which is generally in a direct line with the long axis of the tooth; when 
there is a deviation from this the summit is usually thrown a little to 
the mesial, resulting in a reduction of the length of the mesial cutting-edge. 

As previously stated, the greatest mesiodistal diameter of the surface 
is on a line with the angles of the crown; this measurement is much reduced 
at the cervical line, so that the point of contact with adjoining teeth is 
thrown near the occlusal margins of the crown. This variation in the 
transverse measurement also results in what is commonly referred to as 
the "bell shape" of the crown. The cervical margin of the surface is 
fairly well arched, but seldom to such a degree as the corresponding 
margin upon the incisors and cuspids. 

The Lingual Surface of the Crown (Fig. 81). — This surface is 
smooth and decidedly convex, the absence of strongly developed grooves 
and ridges contributing to the former fact. Like the buccal surface, 
its greatest transverse measure- 
ment is at the base of the cusp in 
which it terminates. The extent 
of the surface is about one-third 
less than that of the buccal, and 
its occluding and cervical margins 
are alone well defined, the mesio- 
distal convexity passing so gradu- 
ally into these respective surfaces 
that a positive line of distinction Mesiomar- 

can scarcely be recognized. In a ' ' . - "" "~~ L cu^p al 

passing from the cervical line to 
the occlusal margin, the surface is Fig. 82.- 

rapidly carried toward the center 
of the crown. The cervical margin of this surface is usually in the form 
of a direct line encircling the neck, but occasionally presents a slight 
concavity in the direction of the root. 

The Mesial Surface of the Crown (Fig. 82). — This surface of the 
crown has three of its borders well defined; these are the buccal, the 
occlusal, and the cervical, the remaining or lingual margin passing so 
gradually into the lingual surface that no positive line of demarcation 
can be given. The buccal margin extends from the mesial angle to the 
cervical line, and invariably presents a slight buccal inclination, thus 
increasing the width of the surface on its cervical portion. The occlusal 
margin is formed by the mesiomarginal ridge, and by a portion of the 
ridge descending from the lingual cusp. It is irregularly V-shaped, 



Cervical 
Line 

Buccal 
Ridge 




-Upper First Bicuspid, Mesial 
Surface. 



u* 



ANATOMY. 



Lingual 
Root 



Buccal Root 



Distomar- 
ginal Ridge 
Lingual 
Cusp 



Buccal 
Ridge 



and in many instances is broken in the center by the mesial groove. The 
cervical margin differs from those of the incisors and cuspids, nearly 
always being in the form of a straight line from buccal to lingual. The 
surface in general is flattened, but shows a slight general convexity near 
the occlusal margin, and frequently a slight concavity immediately below 
the cervical line, this form placing the point of contact with the distal 
surface of the cuspid near the occlusal margin. The surface is occa- 
sionally divided into a buccal and a lingual portion by the mesial groove. 
which may extend to the cervical line, but which generally disappears 

near the center of the surface. 
The buccal half of the surface 
which is formed from the mesial 
developmental lobe is inclined to 
angularity, while the lingual half 
is decidedly rounded, particularly 
in the direction of the occlusal 
margin. 

The Distal Surface of the 
Crown (Fig. 83). — In general, 
this surface resembles the mesial, 
being flattened and bounded by 
three more or less distinct mar- 
gins. The slight buccolingual 
convexity is not confined to the 
occlusal portion of the surface, 
but is inclined to extend to the 
cervical margin, in this particular being at variance with the mesial 
surface. This surface passes into the lingual by a much longer curve 
than that shown on the mesial surface. 

The Angles of the Crown. — These are two in number and, as in 
the teeth previously described, are named, according to their location. 
mesial and distal. The mesial angle is formed by the union of the mesio- 
marginal ridge and mesial cutting-edge. It is primarily the product of 
the mesial developmental lobe, and is usually well produced. The 
distal angle, which is formed in a like manner, is inclined to be more 
rounded. 

The Neck of the Tooth. — In most typal forms the neck of the 
upper first bicuspid is well defined, particularly upon the mesial and 
distal surfaces. Viewing the tooth from a buccal aspect, the neck is 
a distinctive feature, but when studied from the mesial or distal sides, 




Triangular Ridge, 
Occlusal Surface 

Fig. 83. — Upper First Bicuspid, Distal 

Surface. 



UPPER FIRST BICUSPID. I39 

the constriction is scarcely observed, this being particularly the case if 
the tooth has but a single root. In general, the neck partakes of the 
contour of the crown, being convex on the buccal and lingual, and flat- 
tened and frequently slightly concave on the mesial and distal. 

The Roots of the Upper First Bicuspid. — This tooth is usually 
developed with two roots, sometimes with only one, and in rare instances 
it may have three. When two roots are present, one is above the buccal 
and the other above the lingual half of the crown, and are named, accord- 




FiG. 84. — Types of Bicuspids. 

ing to their location, as buccal and Ungual. In general form the two 
roots are quite similar, but the buccal is usually a trifle longer than the 
lingual. They taper off to a slender apex, and are inclined to curve in 
various directions near their extremities. The point of bifurcation is 
frequently some distance above the neck, so that the tooth may be said 
to possess a single root with two branches. Below the bifurcation the 
root assumes the form of the neck or cervical portion of the crown, but 
as the bifurcation is approached, the mesial and distal sides present a 
longitudinal groove, which gradually increases in depth until the single 
root becomes separated. The curves of the root-branches above referred 
to are, in the buccal branch, first to the buccal and then to the lingual; 
while the lingual branch first shows a slight lingual inclination immedi- 
ately above the point of separation, followed by a gentle buccal curve 
as the apical end is reached. In some cases the bifurcation begins 



140 



ANATOMY. 



immediately above the neck of the tooth; in others it may occur in the 
apical third; while a third class is represented by the two roots being 
united throughout their entire length by a thin septum of dentin and 
cementum, or, as occasionally happens, by a layer of cementum alone. 
In this latter instance each root is provided with a distinct canal and 
foramen. When the tooth has but a single root, it is much flattened 
from mesial to distal, the flatness being slightly broken by an inclination 
to convexity. The four surfaces usually converge toward the apical end, 
which is oblong from buccal to lingual, and generally provided with a 

slight distal curve. The presence, of 
three roots is so rare that the condi- 
tion might be classed as a malforma- 
tion; but when they do exist, two are 
usually attached to the buccal and one 
to the lingual half of the crown, with 
the point of separation near the neck 
of the tooth. 

Bilious Type (Fig. 85).— The 
upper first bicuspid of this tempera- 
mental type is marked by a crown of 
moderate length, the neck well pro- 
nounced, and the cusps and angles 
marked by angular outlines. The 
buccal ridge is strongly defined, and 
the buccal grooves, which extend well up on the buccal surface, cross the 
mesial and distal cutting-edges, separating them into two distinct parts. 
The cusps are long and penetrating, and are nearly of equal length, 
assisting to form the firm and well-locked occlusion common to this type. 
The mesial and distal surfaces are nearly parallel with each other; they 
are seldom convex, so that the approximating teeth are in contact over 
an extent of surface rather than a single point. The cervical line is but 
little curved. 

Nervous Type (Fig. 86). — In this temperament the bell-shaped 
crown is strongly observed, the crown being long and much con- 
stricted at its neck. The extreme length of the buccal cusp, and the 
marked cervical constriction, produce an appearance in the buccal 
surface resembling the labial surface of the cuspid tooth. The develop- 
mental grooves are finely outlined, and the cusps long and penetrating, 
usually being more pronounced than in any other class. The cutting- 
edges are sharp and inclined to angularity; the mesial and distal surfaces 




Fig. 85.— Bilious Type, Distal 
Surface. 



UPPER FIRST BICUSPID. 



141 



are convex near their occlusal margins, but near the cervical line a 
pronounced concavity is observed, which is continued upon the correspond- 
ing root-surfaces. This formation forces the point of contact with 
adjoining teeth well toward the occlusal surface, and results in an exten- 
sive V-shaped interproximate space. The cervical line is sharply and 
gracefully formed, the curvature being well arched. 

Sanguineous Type. — The typical upper first bicuspid of this class 
is provided with a crown well proportioned, its length being somewhat 
greater than its breadth, but about equal to its buccolingual measure- 
ment. The buccal surface is seldom broken by the 
buccal grooves, and is strongly convex in every direc- 
tion. The lingual surface is much more rounded 
than the same surface of other types. The mesial 
and distal surfaces are usually smoothly convex, with 
an occasional slight concavity immediately below the 
cervical line. Upon the occlusal surface the grooves 
are rounded and obscure, rather than sharp and well 
defined, and the cusps, much less pronounced than in 
either of the types previously described, are rounded 
and smooth; this latter fact is particularly true of the 
lingual cusp, which is usually much smaller than the 
buccal. The cutting-edges of the buccal cusp are 
scarcely deserving of the name, being broad and 
rounded throughout. The form of the mesial and 
distal surfaces above described provides for a point of contact near the 
center of each surface, leaving a slight interproximate space both above 
and below this point. 

Lymphatic Type. — An examination of the upper first bicuspid of 
the lymphatic type results in finding a tooth vastly different from any 
of those previously described. The length of the crown from the cervical 
line to the point of the cusp is less than either the mesiodistal or bucco- 
lingual measurements. In general appearance it is lacking in symmetry, 
or poorly proportioned. The buccal surface presents a gradual convexity 
from mesial to distal, and seldom has the buccal ridge well developed. 
The lingual surface is smoothly convex and passes off into the lingual 
root without the interposition of a decided neck. The mesial and distal 
surfaces are flattened and sparingly convex, and are nearly parallel with 
each other, so that the contact with adjoining teeth is inclined to be 
distributed over the entire surface, leaving little or no interproximate 
space. The cusps are short, flat, and rounded, and the occlusal surface 




Fig. Ho. — iNerv- 
ous Type, Buccal 
Surface. 



142 ANATOMY. 

much flattened in general, corresponding with the nature of the occlusion, 
which is loose and wandering. The developmental grooves and ridges 
are fairly well shown, while the cutting-edges and angles of the crown 
are smooth and rounded. The neck is less pronounced in this type than 
in any other, the curvature of the cervical line is very slight, the root is 
short and heavy set, frequently passing well up toward the apex before 
bifurcating. 



UPPER SECOND BICUSPID. 143 



UPPER SECOND BICUSPID. 




7th year 8th year 9th year 10th year nth year 12th year 

Fig. 87. 

Calcification Begins, from Four Centers, about the Fifth Year. 
Calcification Completed, Eleventh to Twelfth Year. 
Erupted, Eleventh to Twelfth Year. 
Average Length of Crown, .29. 

Average Length of Root, 55. 

Average Length over All, .84. 

The process of development in this tooth is identical with that of 
the first bicuspid, calcification in the buccal half of the crown taking 
place in one central and two lateral lobes, while the lingual half is developed 
from a single center. The calcifying process is about one year later 
than that in the first bicuspid, the summit of the buccal cusp receiving 
its lime salts about the beginning of the fifth year. During the following 
six months calcification begins in the lateral lobes, and also in the lingual 
lobe. By the sixth year the occlusal surface and a portion of the crown 
are completed by a union of the various lobes, and at seven years the 
crown is calcified for more than two-thirds of its completed length. 
Between the eighth and ninth year the contour of the crown is established, 
and the neck of the tooth and outline of the root-base formed. At the 
tenth year about one-third of the root-length is formed, and during the 
following year about 1/8 of an inch is added to it. By the eleventh or 
twelfth year calcification is completed (Fig. 87). 

This tooth so closely resembles the first bicuspid that a description 
in detail will be unnecessary; there are, however, a few minor points 
which are at variance and must be described in order to distinguish one 
from the other. In general, the tooth is a trifle smaller than the first 
bicuspid, the cusps are somewhat shorter, and the various ridges less 



144 



ANATOMY. 



Central 
Groove 




Mesiomar- 
ginal Ridge 



Fig. 88. — Second Upper Bicuspid 
Occlusal Surf;u e. 



distinct. A distinguishing feature of the occlusal surface is found in the 
diminished length of the central groove (Fig. 88). This groove, as 
observed in the first bicuspid, extends from mesial to distal for fully 
three-fourths of the entire width of the surface; but in the second bicuspid 
it is diminished by one-third, being thus reduced by the broadened 

marginal ridges, which force the mesial and 
distal pits well toward the center. It is 
not uncommon to find the triangular grooves 
joining the central groove directly in the 
Triangular center of the surface, forming a central pit 

Grooves 

from which may radiate numerous small 
supplemental grooves and ridges. The 
summits of both the buccal and lingual 
cusps are nearer to the mesial than to the 
distal surface, thus increasing the length of 
the ridges which descend from them in a distal direction, and decreasing 
those which pass to the mesial. The buccal surface presents a greater 
convexity than that of the first bicuspid; the buccal grooves are usually 
shallow depressions and are frequently entirely wanting, thus giving the 
buccal ridge the appearance of extending its margins to the angles of 
the crown. Unlike the first bicuspid, 
the mesiodistal diameter of the mesial 
surface is but little more than the 
same measurements on the lingual 
surface. The neck of the tooth is 
not quite so pronounced as that of 
the first bicuspid, thus giving less of 
the bell shape to the crown. One 
very important difference between 
this tooth and the first bicuspid is in 
the root-formation. We have seen 
that the first bicuspid is generally pro- 
vided with two roots, while in the 
second bicuspid a single root is 
usually present. Like the first bi- 
cuspid, there are exceptions to this, 

the tooth sometimes being provided with two, and in rare instances with 
three, roots. When the single root is present, it partakes of the form 
of the crown at its base, being well rounded on the buccal and lingual 
portions and much flattened on the mesial and distal. The mesiodistal 



Cervical 
Line 

Buccal 


1 ■ 


Ridge 

Mesiomar- 

ginal Ridge 





Buccal 
Surface 



Lingual 
Cusp 



Fig 89. — Upper Second Bicuspid, Me- 
sial Surface. Most common form. 



UPPER SECOND BICUSPID. 



145 



diameter of the root at its base is only about one-third that of the bucco- 
lingual measurement, this proportionate size continuing throughout 
its entire length. In passing from the base to the apex the root is gradu- 
ally diminished in size, finally ending somewhat 

abruptly in an oblong extremity. In some in- 
stances the apical end is round and pointed, 
resembling the apex of the incisors and cuspids, 
but when thus formed the root is usually curved 
near its apical third and somewhat extended in 
length. The mesial and distal sufaces are pro- 
vided with a well-defined longitudinal concavity, 
extending from the cervical margin to the apex and 
dividing the root into a buccal and a lingual por- 
tion. This depression is often so decided that the 
contour of the single root is almost lost, and in its 
place the appearance is that of two roots similar to 
those described in the first bicuspid. The length 
of the root is usually a little greater than that of 
the first bicuspid, but the crown being a trifle 
shorter, results in producing a tooth the entire 
about equal to that of the first bicuspid. 




Fig. 90. — Young Up- 
per Second Bicuspid, 
Lingual Surface. 



length 



of which is 



146 



ANATOMY. 



UPPER FIRST MOLAR. 



g>~& ft V f 



2d year 3d year 5th year Gth year 7 th year 

Fig. 91. 



10th year 



1 1 th vear 



Calcification Begins, from Four Centers, about Onk Month Before Birth 
Calcification Completed, Ninth to Tenth Year. 
Erupted, Sixth to Seventh Year. 
Average Length of Crown, .30. 

Average Length of Root, .51 

Average Length over All, 81 

This tooth being the first of the permanent organs to erupt, it 
precedes all others in the process of calcification, beginning to receive 
its lime salts as early as the eighth fetal month. The form of the crown 
being so entirely different from those previously described, embodies 
a developmental process which is also different, four distinct lobes being 
present, one for each cusp, these making their appearance during the 
first year after birth, closely followed by a completion of the occlusal 
surface by the union of the free calcifying margins, these lines of union 
being finally represented by the developmental grooves of the occlusal 
surface. After the completion of this surface, calcification proceeds in 
the direction of the base of the crown, and at the beginning of the third 
year about two-thirds of the crown is formed. During the fifth year 
(he contour of the crown is completed, and at the beginning of the eruptive 
period, or about the sixth year, the developmental process has extended 
to the base of the roots, and an effort at trifurcation begun. At seven 
years the three roots with which the tooth is provided are branching out, 
each into its own socket, subsequent development in each being con- 
tinued as a separate and distinct process. The tenth year more than 
half completes the calcifying process in the roots, and at the beginning 
of the eleventh year the root apices are formed. Like the first bicuspid, 
the crown of this tooth presents for examination five surfaces — occlusal, 
buccal, lingual, mesial, and distal. In general contour it i^ irregularly 



UPPER FIRST MOLAR. 147 

quadrilateral, with the angles of the crown more or less rounded, two 
of its sides convex and two flattened or slightly concave. The length 
of the crown from the cervical line to the summits of the cusps is about 
equal to, or slightly less than, its mesiodistal diameter, while the bucco- 
lingual measurement is usually a trifle greater than the mesiodistal. 

The Occlusal Surface of the Crown (Fig. 92). — The coronal 
outline of this tooth is best studied when looking directly upon this 
surface, which shows the two convex sides above referred to, represented 

Mesiobuccal Cusp 



Buccal Groove 



Central Fossa B ■ Dist ° buccal 

^^ ^K Cusp 

Oblique Ridge ^" f 1 I Distal Fossa 




Mesiolingual Cusp W^. ^T 'B Dist P5' a i'" 

The Fifth Cusp ^^^ ^^ 



Distolingual Distolingual 
Groove Cusp 

Fig. 92. — Upper First Molar, Occlusal Surface. 

by the buccal and lingual margins, with the mesial and distal margins 
more or less flattened. The surface is bounded by these four margins, 
which are nearly of equal length, the angles formed by their union being 
more or less rounded, two of which, the mesiobuccal and the distolingual, 
are acute angles, while the mesiolingual and distobuccal are obtuse angles. 
The surface is divided into four developmental portions — the mesiobuccal, 
distobuccal, mesiolingual, and distolingual. Each one of these parts 
is surmounted by a well-defined point or cusp, which likewise is named 
in accordance with its location. These various parts are separated 
from one another by four developmental grooves — the mesial, the buccal, 
the distal, and the distolingual. In the center of the triangle formed 
by the central incline of the mesiobuccal, distobuccal, and mesiolingual 
cusps is a deep depression — the central fossa — while near the distal 
margin is a somewhat similar depression — the distal fossa. Traversing 
the surface in various directions are a number of ridges and supplemental 
grooves, each of which will be described in turn. 

The Marginal Ridges of the Occlusal Surface. — These are four in 
number — the mesial, distal, buccal, and lingual. The mesiomarginal 
ridg£ is a well-prortounced elevation of enamel which passes from the 
mesiobuccal angle to the mesiolingual angle. It is slightly concave in 



148 ANATOMY. 

the direction of the root, and is broken near the center of its concavity 
by the mesial groove, upon either side of which are frequently found one 
or two small points or tubercles, which are formed either by a division 
of the mesial developmental groove, or by one or more supplemental 
grooves. These grooves pass over the ridge and are continued for a short 
distance on the mesial surface. Descending from the mesiobuccal cusp, 
the ridge passes in a lingual direction to meet the mesiolingual cusp, and 
in so doing has a slight distal inclination until the mesial groove is reached, 
after passing which it makes a sweeping distal curve and is lost in the 
lingual margin. This ridge marks the line of junction between the 
occlusal surface and the mesial surface. The distomarginal ridge in 
some respects resembles the mesial just described, being concave and 
ascending in a buccal and lingual direction, with a somewhat rounded 
outline, to the summits of the distobuccal and distolingual cusps. The 
depth of the concavity is usually greater than that of the mesial margin, 
and is frequently crossed near the center by the distolingual groove, 
frequently so marked as to produce a V-shape to the center of the margin. 
There are occasionally found upon either side of this central groove one 
or more small tubercles, corresponding to those of the mesial ridge, but 
they are less frequent and less pronounced. This ridge forms the line 
of demarcation between the occlusal surface and the distal surface. The 
buccomarginal ridge begins at the mesiobuccal angle, and gradually 
ascends to the summit of the mesiobuccal cusp, from which it afterward 
descends in a distal direction to the buccal groove; continuing, it again 
ascends the distobuccal cusp, after descending from which it ends in the 
distobuccal angle. The nature of this ridge is a series of cutting-edges, 
giving to the cusps their angular nature. Besides the buccal groove, 
which makes a decided break in the center of its course, the ridge is 
frequently crossed by numerous small supplemental grooves occurring 
in various locations and forming a series of minute tubercles; this latter 
condition is most frequently present in young teeth, and is soon obliterated 
by wear. The course of this ridge is not that of a direct line from mesial 
to distal, but in its ascent of the mesiobuccal cusp it is inclined to the 
buccal; in its descent it presents a corresponding return to the lingual, 
and the same variations are observed in passing over the distal cusp. 
The linguo marginal ridge begins at the mesiolingual angle of the crown 
and passes distally to the distolingual angle, differing from the three 
previously described by being heavy and rounded in its nature, more 
irregular in outline, and divided nearest to its distal extremity instead 
of in the center of its length. From the point of beginning it makes a 



UPPER FIRST MOLAR. 149 

curved ascent to the summit of the mesiolingual cusp; descending from 
this in a distobuccal direction, it divides, one portion passing to join the 
triangular ridge of the distobuccal cusp, the two uniting to form the 
oblique ridge, the other portion continuing in the direction of the disto- 
lingual cusp, before reaching the base of which it is broken by the disto- 
lingual groove. From this groove the ridge makes a sudden and direct 
ascent to the summit of the distolingual cusp, after passing which it 
gradually descends in a long curve to join the distomarginal ridge. Like 
the buccal ridge, it is frequently crossed by numerous supplemental 
grooves. The ridge forms the lingual margin of the occlusal surface, 
and gives to the cusps their angularity. 

The Cusps (Fig. 92). — These are four in number — the mesiobuccal, 
distobuccal, mesiolingual, and distolingual. 

The Mesiobuccal Cusp (Fig. 92). — In extent of surface this is usually 
the largest cusp, although it is sometimes. exceeded by the mesiolingual. 
From the summit of the cusp three ridges descend — the buccal ridge to 
the buccal surface, the buccomarginal ridge making a double descent, 
and the mesiobuccal triangular ridge, the latter descending the central 
incline and ending in the central fossa. The mesial base of the cusp is 
frequently crossed by one or more supplemental grooves, which begin at 
the mesial margin and pass in the direction of the central fossa. The 
central slope of the cusp contributes to the formation of the central fossa, 
its extent in this direction being controlled by the mesial and buccal 
grooves, which together form the mesiobuccal triangular groove. 

The Distobuccal Cusp (Fig. 92). — This cusp is frequently the smallest 
in extent of surface, but is usually longer and more pointed than the others. 
Like the mesiobuccal cusp, three ridges descend from it — the buccal 
ridge to the buccal surface two which spring from the buccomarginal 
ridge, and the distobuccal triangular ridge, which descends obliquely 
toward the distal center of the surface and joins a similar ridge (pre- 
viously described) from the mesiolingual cusp, the two forming the 
oblique ridge. The mesial portion of the base of this cusp assists in 
forming the central fossa, while a portion of the distal contributes to the 
formation of the distal fossa. The inner boundary of the cusp is formed 
by the buccal groove, the distal groove, and by a portion of the disto- 
lingual groove. 

The Mesiolingual Cusp (Fig. 92). — As above stated, this cusp is 
frequently the largest in extent of surface, and is somewhat rounded, 
with its summit poorly defined. The ridges which descend from it 
correspond in name and number to those of the buccal cusps, the linguo- 



150 ANATOMY. 

marginal ridge making a double descent, the mesiolingual ridge descend- 
ing to the lingual surface, while the central incline is marked by the 
mesiolingual triangular ridge, which ends in the central fossa. Toward 
the mesial portion of the cusp one or more small ridges are frequently 
present, extending from the marginal ridge to the mesial groove. The 
distal descent of the marginal ridge is bifurcated, one portion making 
a sweeping curve and joining the transverse ridge from the buccal cusp, 
forming the oblique ridge previously referred to, the other portion 
passing in a distal direction and ending at the distolingual groove. The 
central incline of this cusp forms the lingual side of the central fossa, 
and its boundaries are outlined by the mesial, distal, and distolingual 
grooves. 

The Distolingual Cusp (Fig. 92). — This cusp is usually the smallest 
of the four; it is triangular in outline, with the summit nearest the mesio- 
lingual portion. The ridges which descend from this cusp are only two 
in number, one passing in a mesial direction and forming a portion of the 
linguomarginal ridge, the other passing to the distal, with a gradual 
buccal curve, to join the distomarginal ridge. Of the two remaining 
inclines, one looks in a distolingual direction, presenting a surface which 
is smooth and rounded; the other, sloping by a broad expanse in a mesio- 
buccal direction, ending in the distolingual groove, and also assisting to 
form the distal fossa. This latter incline is often crossed by small sup- 
plemental grooves, which take a winding course from the base to the 
summit of the incline. The inner margin or outline of the cusp is formed 
by the distolingual groove. 

The Fifth Cusp (Fig. 92). — Although usually referred to as possessing 
but four cusps, this tooth is frequently developed with five, the additional 
lobe being situated on the lingual side of the mesiolingual cusp, about 
midway between its summit and the neck of the tooth. When present, 
it is distinctly separate from the main cusp by a well-developed groove — 
the mesiolingual groove. Both the cusp and the groove may be more or 
less developed, the former in some instances assuming dimensions cor- 
responding to that of the distolingual cusp, and the latter sometimes 
being as well marked as the distolingual groove. When thus pronounced, 
the groove begins near the center of the mesial surface, and passes ob- 
liquely toward the summit of the mesiolingual cusp, before reaching which 
it makes an abrupt turn rootward, and joins the lingual terminal of the 
distolingual groove, this union frequently resulting in a well-defined pit— 
the lingual pit. This cusp, as usually found, is small and apparently 
without function. When occurring on the tooth of one side, it is usually 



UPPER FIRST MOLAR. 



IU 



present on the corresponding tooth of the opposite side. It is seldom 
present on any but the upper first molar. 

The Fossae and Grooves of the Occlusal Surface (Figs. 92 and 
93). — The fossae are two in number — central and distal. The central 
jossa occupies a position near the center of the surface, and is formed by 
the central incline of the mesiobuccal, distobuccal, and mesiolingual 
cusp, which usually give it a three-sided form. Connecting the three 
sides of the fossa, and in a measure assisting in its construction, is the 



Buccal Groove 




Distobuccal 
Cusp 

Distal Groove 



Distolingual 
Cusp 



Mesiobuccal 

Cusp 

Central Fossa 

Mesiomar- 
ginal Ridge 

The Fifth 
Cusp 



Mesiolingual Oblique 
Cusp Ridge 

Fig. 93. — Upper First Molar, Occlusal Surface, Strongly Developed, showing 

Presence of Fifth Cusp. 

mesiomarginal ridge and the oblique ridge. The depth of this fossa, 
as well as that of the distal, is of course regulated by the length of the 
cusps, which in turn is much influenced by the temperamental type of the 
tooth. The bottom of the fossa is deeply marked by two of the grooves 
of development — the mesial groove and the buccal groove. The former 
begins on the mesial surface, passes over the mesiomarginal ridge, and 
continues in an irregular line to the bottom of the fossa; the latter, begin- 
ning near the center of the buccal surface, enters the fossa by crossing 
the buccomarginal ridge near the center of its length, and also ends in 
the central pit of the central fossa. As previously referred to, the union 
of these two grooves forms the mesiobuccal triangular groove. From 
the central pit of this fossa another groove is given off — the distal groove. 
It is usually well defined at its beginning, but as it passes over the oblique 
ridge it is generally partly obliterated, although occasionally being so 
marked as to divide this ridge. The distal fossa is much smaller than 
the central, and is of an entirely different form. Its walls are principally 
formed by the distolingual incline of the oblique ridge, and the mesio- 
buccal incline of the distolingual cusp; a portion of the distomarginal 



152 



ANATOMY. 



Distobuccal 
Root 



Mesiobuccal 
Root 



ridge and the distal incline of the distobuccal cusp also assist in its forma- 
tion. Like the central fossa, its sides are more or less irregular, from 
the presence of various grooves and ridges in its vicinity. The greatest 
length of the fossa is in a distolingual direction, and it is traversed by 
a deep developmental groove — the distolingual groove. When the distal 
groove crosses the oblique ridge, it usually extends to the bottom of this 
fossa. 

The Buccal Surface of the Crown (Fig. 94). — This surface, which 
is the result of a union between the mesial and distal developmental 

lobes, may be divided into a 
mesial and a distal half. These 
two portions are quite similar 
in outline, and are separated 
J Lingual Root from each other by the buccal 
groove, which usually ends near 
the center of about half-way to 
the cervical line in a decided 
pit — the buccal pit. In some 
instances this groove is con- 
tinued to the cervical line, or 
even beyond this to the bifur- 
cation of the roots. Both the 
mesial and distal half are pro- 
vided with a longitudinal ridge 
(the buccal ridges) — one the 
mesiobuccal ridge and the other 
the distobuccal ridge. These are 
similarly formed and descend from the summits of the respective cusps, 
at which point they are usually well denned, but gradually disappear as 
they pass toward the cervical line. The location of the buccal groove being 
a little to the distal of the center of the surface, gives to the mesial portion 
a somewhat greater extent than the distal. The margins of the surface, 
which form an irregular quadrilateral, are the mesial, distal, occlusal, and 
cervical. The mesial and distal margins are rounded, and gradually con- 
verge as they pass rootward, making the average diameter of the surface 
about one-fourth less at the cervical line than at the base of the cusps. 
In some instances these margins are slightly concave over their cervical 
portion, and convex on approaching the occlusal margins; or the mesial 
may be concave and the distal convex throughout their entire length; 
in some types they appear as straight lines and are parallel with each 



Distobuccal 
Cusp 



FlG. 94. 




Mesiobuccal 
Cusp 



Buccal Groove 

-Upper First Molar, Buccal 
Surface. 



UPPER FIRST MOLAR. 



J 53 



Distobuccal 
Root 



Mesiobuc- 
cal Root 



other. The occlusal margin is formed by the marginal ridges as they 
pass over the two buccal cusps, being in the form of the letter W. The 
cervical margin is usually a direct line drawn around the circumference 
of the tooth, but in some instances deviating slightly from this. Immedi- 
ately below the cervical line, and conforming to its general direction, 
is a rounded fold of enamel — the cervical ridge. 

Lingual Surface of the Crown* (Fig. 95). — In correspondence 
with the buccal surface, this surface is developed from two lobes — 
the -mesio- and disto-lingual lobes — the line of union between the two 
being recorded by a well- 
defined groove — the lingual 
groove. This groove, which 
is a continuation of the dis- 
tolingual groove of the occlusal 
surface, usually ends near the 
center of the lingual surface in 
a well-defined pit — the lingual 
pit — or it may continue root- 
ward and gradually disappear. 
It is located a little to the 
distal of the center of the sur- 
face, thus making the mesial 
a trifle larger than the distal 
portion. The mesial half of 
the surface is smooth and 

convex; the lingual incline of the mesiolingual cusp is seldom provided 
with a well-defined ridge, although usually referred to as the mesio- 
lingual ridge. The distal half of the surface is also smooth and 
rounded, with the mesiodistal convexity much more marked than that 
of the mesial lobe. The cervical ridge is seldom so pronounced as 
that of the buccal surface, but the enamel frequently makes a 
sudden dip at this point to meet the cementum of the root. That 
portion of the surface immediately below the cervical ridge is smooth 
and unbroken, slightly convex in the direction of the long axis of the 
tooth, and flattened or slightly convex from mesial to distal. The 
margins of the surface are the mesial, distal, occlusal, and cervical. 
The surface passes so gradually into the mesial and distal that it is 
somewhat difficult to define these margins. In general, the margins 



Distolingual 
Angle 



Fig. 95. 




Lingual 
Groove 



Lingual Ridge 
Mesiolingual Cusp 



-Upper First Molar, Lingual 
Surface. 



*When the fifth cusp is present, the anatomy of the mesial half of this surface is some- 
what more complex. 



154 



ANATOMY. 



Lingual 
Root 



Cervical 
Line 



Mesiolingual 
Angle 



Mesiobuc- 
cal Root 



converge slightly in the direction of the root. Both the occlusal and 
cervical margins are similar to the corresponding margins of the buccal 
surface. 

The Mesial Surface of the Crown (Fig. 96). — This surface is 
almost an unbroken plane, being smooth and flat. In some instances 
it is crossed near the center of its occlusal margin by a continuation of 
the mesial groove, but this is seldom so pronounced as to divide the 
surface. The occlusal third of the surface is inclined to a slight general 
convexity, providing a point of contact for the distal surface of the second 

bicuspid, but between this 
and the cervical line there is 
often a slight concavity. The 
margins of the surface are the 
occlusal, buccal, lingual, and 
cervical. The first named is 
formed by the mesiomarginal 
ridge of the occlusal surface, 
and is concave in the direc- 
tion of the root. The buccal 
and lingual margiris are 
rounded, and, unlike the 
lateral margins of the buccal 
and lingual surfaces, diverge 
in the direction of the roots. The cervical margin is slightly concave in 
the direction of the occlusal surface, and its length is much greater than 
that of any other margin of the crown. This surface is more extensive 
than either the buccal, lingual, or distal, and is about equal to that of 
the occlusal. When the fifth cusp is present it alters the form of the 
lingual margin of this surface by crossing it near the center, and ex- 
tending for some little distance on the face of the surface. 

The Distal Surface of the Crown (Fig. 97). — Taken in its entirety, 
this surface usually presents a general convexity. The lingual half of 
the surface is usually somewhat more prominent than the buccal, the 
latter being flattened and frequently slightly concave, particularly near 
the cervical portion. In some instances the surface is traversed by a 
continuation of the distolingual groove, which, after passing over the 
distomarginal ridge, is continued in a longitudinal direction, dividing 
the surface into two equal parts. Not infrequently this groove, instead 
of existing as such, is represented as a shallow depression, often extend- 
ing to the bifurcation of the roots. The margins of the surface are four 




Mesiobuc- 
cal Angle 



Fig. 96. — Upper First Molar, Mesial 
Surface. 



UPPER FIRST MOLAR. 



r 55 



Lingual 
Root 



Mesiobuc- 
cal Root 

Distobuc- 
cal Root 



Disto- 

lingual 

Cusp 



Distobuc- 
cal Cusp 



in number: the occlusal, which closely resembles the corresponding 
margin of the mesial surface; the buccal, which is not well defined; the 
lingual, somewhat angular; and the cervical, formed by the cervical line. 

The Neck of the Tooth. — When looking upon the buccal surface 
of this tooth, the constricted portion forming the neck is greatest at a 
point immediately above the cervical line. Viewed in this direction the 
crown is usually bell-shaped, and both the crown and the base of the 
roots assist in producing the neck. Viewed from a lingual direction, 
the neck is a distinctive feature, but is seldom so marked as when ex- 
amined from the opposite side. 
When studied from either a mesial 
or a distal aspect, the neck appears 
above the cervical line, the promi- 
nent fold of enamel immediately 
adjacent to this line forcing the 
neck rootward. 

The Roots of the Upper First 
Molar. — The roots of this tooth are 
three in number, two of which are 
on the buccal side, and are, there- 
fore, called mesiobuccal and disto- 
buccal roots, and one on the lingual 
side, known as the lingual root. 
These three roots are given off 
from a common base, which is 

sometimes referred to as the root, while those parts above the point of 
trifurcation are considered as root-branches. The number, location, 
and form of the roots of this tooth are, perhaps, more constant than those 
found in connection with any other cuspidate tooth. The common base 
from which the roots are given off is similar in contour to the crown of 
the tooth, excepting in those cases in which the form of the root is carried 
over this base to meet the neck of the tooth. 

The mesiobuccal root (Fig. 97) is flattened from mesial to distal, broad 
at its base from buccal to lingual, from which point it gradually tapers 
to the apex. At the base the mesiodistal measurement is less than one- 
third that of the buccolingual. In its course it is first inclined to the 
mesial, but after reaching the center of its length it makes a decided 
distal curve, which looks almost directly to the distal. The mesial side 
of this root is decidedly flattened at its base, but as the center of the 
surface is reached a shallow longitudinal groove is present, which is 




Distolingual Groove 

Fig. 97. — Upper First Molar, Distal 
Surface. 



156 ANATOMY. 

continued to the region of the apex. The distal side is also possessed 
of a similar groove, which extends throughout its entire length. Both 
the buccal and lingual sides of the root are smoothly convex, the latter 
being only about half the width of the former. 

The distobuccal root (Fig. 97) is much the smallest of the three, and, 
while inclined to flatness on its mesial and distal sides, it is much more 
rounded than the mesial root. The mesial side is provided with a slight 
longitudinal groove, and in rare instances a similar groove exists on the 
distal side. The buccal and lingual sides are similar to those of the 
mesial root. The root is generally straight, and tapers gradually from 
base to apex, ending in a rounded point. 

The lingual root (Fig. 97) is usually the largest and longest of the 
three, and is more rounded in form than either of the buccal roots. The 
lingual surface is inclined to flatness near its base, and is provided with 
a well-defined longitudinal groove, which is sometimes independently 
formed, while at others it is present as a continuation of the lingual 
groove. This root being the only one given off from the lingual side of 
the tooth, is constructed with a mesiodistal measurement about equal 
to that of the base of the crown at this point. From its place of beginning 
it passes first in a lingual and then in a buccal direction, forming a long 
curve and ending in a sharp-pointed apex. 

Bilious Type. — The upper first molar of this temperament is 
manifest by a crown with angles well produced, the marginal ridges and 
cutting-edges of the cusps bold and well marked. The cusps are of 
medium length, with summits angular and pointed. The developmental 
grooves are deep and often sulcate, and numerous supplemental grooves 
are found upon the occlusal surface. The longitudinal and transverse 
measurements of the crown are about equal, and when viewed upon the 
occlusal surface, the angular nature of its anatomy is noted as a distinc- 
tive feature. The neck is fairly well developed, giving a slight bell- 
shape to the crown. The cervical line is made up of angles rather than 
curves, and the roots are long and straight. 

Nervous Type. — Like the teeth previously described under this 
class, the crown of this tooth is of greater longitudinal than transverse 
extent; the neck is especially well formed, producing a decided bell- 
shape to the crown. The cusps are long and penetrating, the marginal 
ridges sharply defined, as are also those ridges upon the central incline 
of the cusps. The grooves of development are decided and frequently 
sulcate. The buccal surface is rounded and smooth, with the buccal 
groove extending well toward the cervical line. The lingual surface 



UPPER FIRST MOLAR. 1 57 

also presents a general convexity, and is usually divided by the lingual 
groove. The mesial surface is convex over its cervical third, and the 
occlusal margin is a decided convex ridge, serving as a point of contact 
for the adjoining tooth, and thus forming the characteristic V-shape 
common to this temperament. It is in this type that the fifth cusp is 
most frequently present. The cervical line is much curved, and the 
roots are slim and frail. 

Sanguineous Type. — The crown of the upper first molar of this 
type usually presents a slightly greater longitudinal than transverse 
extent. The angles of the crown are poorly formed, being rounded and 
smooth. The cusps are of moderate length, and are rounded in their 
nature; the marginal ridges, as well as those ridges of the central incline 
of the cusps, are less distinct than either of the forms previously described. 
The buccal and lingual surfaces are convex and seldom broken by grooves; 
the mesial and distal surfaces are convex in every direction, throwing 
the point of contact with adjoining teeth near the center of the surface. 
The roots are inclined to be large and oval in form, while the cervical 
line is a series of long curves. 

Lymphatic Type. — In this temperament the crown is much less 
in its longitudinal than transverse measurement. The neck of the tooth 
is poorly defined, the crown passing into the root-base without a marked 
constriction. The mesial and distal surfaces are flattened and nearly 
parallel with each other, providing a broad contact surface. The buccal 
and lingual surfaces each present a marked general convexity, the latter 
being frequently broken by the distolingual groove. The tooth is 
provided with cusps which are short and heavy-set; the marginal ridges, 
as well as all the ridges common to the occlusal surfaces, are poorly defined. 
The developmental grooves are shallow and terminate abruptly. There 
is but little curvature to the cervical line, and the roots are short, heavy- 
set, and inclined to cluster together. 



i5« 



ANATOMY. 



UPPER SECOND MOLAR. 



&-& F f 



7th yeat 8th year 9th year 



10th year 11th year 12th year 
Fl(i. 98. 



1 6th year 



Calcification Begins, from Four Centers, about the Fifth Year. 
Calcification Completed, Sixteenth to Eighteenth Year 
Erupted, Twelfth to Fourteenth Year. 
Average Length of Crown, .28. 

Average Length of Root, .51. 

Average Length Over All, 79. 



Calcification in this tooth takes place in precisely the same manner 
as that of the first molar, but the formative process is much later in begin- 
ning, the lime-salts commencing to accumulate in the four separate lobes 
about the fifth year. At the beginning of the sixth year the formation 
of the cusps is completed, soon after which they coalesce and the occlusal 
surface of the crown is established. At the beginning of the eighth 
year fully two-thirds of the crown is calcified, and the following year 
the crown and neck are completed and the root-base outlined. By the 
tenth year the beginning of separate root-development is observed; 
at the twelfth year, or at the time of eruption, the roots are formed to 
about one-half of their completed length, the process continuing until 
the sixteenth or seventeenth year, when calcification is completed and 
the root apices formed (Fig. 98). In many respects this tooth closely 
resembles the first molar previously described, the crown presenting 
the same number of surfaces similarly named, and also being provided 
with the same number of roots. Notwithstanding this fact, there are 
a number of ways in which they are at variance. The crown of the 
second molar is smaller than that of the first, and the quadrilateral 
outline common to the first molar is much compressed and broken in 
the second. The distal cusps are much smaller proportionately than 
the mesial cusps, this being particularly true of the distolingual cusp. 



UPPER SECOND MOLAR. 



J 59 



This reduction in size of the distal cusps gives to that portion of the 
occlusal surface a slight distal incline. 

Occlusal Surface of the Crown (Fig. 99). — The general contour 
of the' crown is best studied by viewing it directly upon the occlusal 
surface; this aspect also shows to best advantage the difference in form 
between this and the first molar tooth, as shown in figure 93. The 
mesial and lingual outlines closely resemble the corresponding outlines 
on the first molar, but the buccal and distal are much at variance, the 
former passing into the latter without a distinct line of demarcation 
existing between the two, this gradual blending of one into the other 
being at the expense of the distobuccal angle of the crown, which is poorly 



Buccal Groove 



Distobuccal Cusp 

Distal Fossa 

Distolingual Groove 




Mesiobuccal 
Triangular Ridge 

Mesial Groove 

Mesiomarginal Ridge 

Mesiolingual Cusp 



Central Distal Incline of 
Fossa Mesiolingual Cusp 

Fig 90 — Upper Second Molar, Occlusal Surface. 

developed. The crown is much compressed in a distobuccal-mesio- 
lingual direction, making this measurement of the occlusal surface about 
one-third less than the mesiobuccal-distolingual measurement. The 
cusps are much inclined to cluster toward the center of the surface, this 
being especially true of those on the lingual half. 

Marginal Ridges of the Occlusal Surface (Fig. 99). — Like the occlusal 
surface of the upper first molar, this surface of the second molar is 
bounded by four marginal ridges — the mesial, distal, buccal, and lingual. 
They are usually less marked than those found on the first molar, and 
are much more variable in their individual anatomy. The mesiomarginal 
ridge extends from the summit of the mesiobuccal cusp to the summit 
of the mesiolingual cusp. It is concave in the direction of the body of 
the crown, and is broken near its central portion by the mesial groove. 
In some instances one or more small supplemental grooves are found 



l6o ANATOMY. 

to cross it. Compared with the mesiomarginal ridge of the first molar, 
its length is much less and the convexity not so pronounced. The 
distomarginal ridge, owing to the variation in form and size of the distal 
cusps, is difficult to describe definitely; suffice it to say that it extends 
from the summit of the distobuccal to the summit of the distolingual 
cusp. The concavity is V-shaped, and is usually crossed near the 
center by the distolingual groove. In some instances the distolingual 
cusp is almost wanting, in others the distobuccal is but little developed; 
when either of these conditions is present, the marginal ridge is extended 
either to the buccal or to the lingual, in a measure taking the place of 
the missing cusp. The mesial half of the buccomarginal ridge closely 
resembles the corresponding margin of the first molar; beginning at 
the mesiobuccal angle it ascends to the summit of the mesiobuccal 
cusp, after which it descends by a longer incline to the buccal groove. 
The distal half of the ridge, unlike that of the first molar, presents much 
variety, its form being controlled by the character and position of the 
distobuccal cusp, usually small. As most frequently observed, it ascends 
to the summit of the cusp, and in so doing it presents a decided lingual 
inclination. In passing down the distal incline the lingual inclination 
is increased and gradually passes into the distomarginal ridge. Branch- 
ing off from the mesiomarginal ridge, the linguo marginal ridge ascends 
to the summit of the mesiolingual cusp and descends by a much shorter 
incline to the distolingual groove. This portion of the margin is thrown 
well toward the center of the surface, the location of the cusp carrying 
it to that point. Like the distal half of the buccal margin, the outline 
of the distal half of this margin is controlled by the position and form of 
the distolingual cusp. In the majority of cases, when the cusp is moder- 
ately strong, the ascent from the distobuccal groove to the summit of 
the cusp is short and abrupt, the descent being somewhat more gradual, 
and with a decided buccal inclination it passes into the distomarginal 
ridge, or ends abruptly at the distal end of the distolingual groove. 

The Cusps and Ridges (Fig. 99). — This tooth is provided with 
four lobes or cusps, two of which are located on the buccal, and two on 
the lingual side. They are usually smaller and less angular than the 
cusps of the first molar. This is particularly true of both the distal 
cusps, and especially of the distolingual cusp, which is often quite diminu- 
tive and occasionally entirely wanting. When this latter condition 
exists, the lingual half of the surface is for the most part occupied by 
what would otherwise be the mesiolingual cusp, the absence of the 
distal cusp permitting the distolingual groove to occupy a position near 



UPPER SECOND MOLAR. l6l 

the extreme distolingual angle, that portion of the surface which is 
distal to the groove being a portion of the distomarginal ridge. 

The Mesiobuccal Cusp (Fig. 99). — Like the corresponding cusp of 
the first molar, this cusp is usually the longest of the four, and in many 
instances covers a greater extent of surface than any of the others. Its 
base is outlined by the buccal and mesial grooves, the two together form- 
ing the mesiobuccal triangular groove. Descending from its summit 
to the buccal surface is the mesiobuccal ridge; the marginal ridge makes 
a double descent, one in a mesial and one in a distal direction, while 
sloping toward the central fossa is the mesiobuccal triangular ridge. 
The cusp is seldom traversed by supplemental grooves such as are found 
on the corresponding cusp of the first molar. 

The Distobuccal Cusp (Fig. 99). — As previously stated, this cusp is 
not constant in its form; in some instances it is bold and well produced, 
corresponding closely to the mesiobuccal cusp just described. When 
thus pronounced it is possessed of ridges, and bounded by grooves which 
are similar to those described in connection with the first molar. More 
frequently the cusp is much rounded, its summit being carried well 
toward the center of the surface. When this formation exists, the buccal 
ridge is absent, the marginal ridges short and rounded; the distobuccal 
triangular ridge which descends from it toward the center of the crown, 
is short and heavy set. 

The Mesiolingual Cusp (Fig. 99). — In the majority of instances this is 
the largest cusp, particularly when there is a degenerate tendency in the 
distolingual cusp. Descending from its summit are a number of ridges, 
the marginal ridges being given off as already described, the mesiolingual 
triangular ridge descending the central incline to the central fossa, and when 
the cusp has an additional mesiodistal extent by the presence of a diminutive 
distal cusp, other ridges descend in the same direction. The lingual 
descent of the cusp is smooth and more rounded than the corresponding 
surface of the first molar, and is seldom elevated in the form of a definite 
ridge. The central outline of this cusp is marked by the mesial, the 
distal, and the distolingual grooves. 

The Distolingual Cusp (Fig. 99). — In no other cusp do we find such 
a diversity of form as in the distolingual cusp of the upper second molar. 
In some instances it is fully as prominent as its neighbor just described, 
in others appearing as a mere fold of enamel, and it is not uncommon 
to find it entirely wanting, the distomarginal ridge extending to occupy 
a portion of the space which it should claim. Deductions might be 
drawn from an average between these two extremes, wherein the existing 



1 62 ANATOMY. 

cusp would be much smaller than any of the others, the summit rounded 
rather than sharp, but with a decided inclination to occupy the extreme 
distolingual angle of the surface, in this latter respect differing from the 
distobuccal cusp. The mesial and buccal outlines of the cusp are formed 
by the distolingual groove, and its mesiolingual incline contributes to 
the formation of the distal fossa. 

The Fossae and Grooves of the Occlusal Surface (Fig. 99). — 
These in name, number, and general form are similar to those of the 
first molar. The central fossa is never, strictly speaking, in the center 
of the surface, and is formed by the central incline of the mesiobuccal, 
distobuccal, and mesiolingual cusps. It is seldom so deep as the central 
fossa of the first molar. The distal fossa is more or less pronounced, 
its size and position being controlled by the extent of development in 
the distolingual cusp. The distolingual groove, which usually crosses 
the lingual surface of the first molar near its center, is not constant in 
its location on this tooth, in some cases being near the center, in others 

near the distolingual angle of the crown. The 
buccal groove is never constant in its location, 
usually crossing the buccomarginal ridge and pass- 
ing over the buccal surface near its mesiodistal 
center, but it is not uncommon to find it forced to 
the distal by a diminution in the size of the distal 
cusp. 

Buccal Surface of the Crown (Fig. 100). — 
The most constant difference between this and the 
corresponding surface of the upper first molar is 
the wandering location of the buccal groove. 
While in the majority of instances it may be found 
,, ,- near the mesiodistal center of the surface, it is not 

Fig 100. — L p p e r 

Second Molar, But- uncommon to find it passing over the distal third, 
or even as far posterior as the distobuccal angle. 
In general, the surface is somewhat more convex and necessarily less 
extensive than the buccal surface of the first molar. The buccal ridges 
which descend from the summit of the two buccal cusps are seldom so 
marked as those on the first molar, and in many instances the distal 
ridge is wanting. The distal half of the surface frequently passes 
into the distal surface, by a long gradual sweep, there being no line of 
demarcation between the two. 

The Lingual Surface of the Crown (Fig. 101). — In keeping with 
the other surfaces just described, the lingual surface differs from the 




UPPER SECOND MOLAR. 



163 



Lingual 
Root 



Mesial In- 
cline of 

Mesiolin- 
gual Cusp 




Cervica 
Line 



Disiomar- 
ginal Ridge 



Fig. ioi. 



Lingual Groove, ending 
in Lingual Pit 

-Upper Second Molar, Lingual 
Surface. 



corresponding surface of the first molar in that it presents a greater 
general convexity. This is particularly true in passing from the cervical 
line to the occlusal surface. The lingual groove is also less constant 
in its location. In most in- 
stances it is to be found a 
little to the distal of the cen- 
ter, in others being as far 
posterior as the extreme distal 
third of the surface, and in 
rare instances it is entirely 
wanting. The general char- 
acter of this surface, which is 
smooth and convex, is seldom 
broken by the presence of 
well-defined ridges, such as 
are usually found descending 
from the lingual cusps of the 
first molar. As previously 
referred to, the mesial, distal, and buccal surfaces, as well as the surfaces 
under consideration, are proportionately smaller than those of the first 
molar, and, while this refers to both the transverse and longitudinal 
measurements, it is particularly applicable to the latter. 

The Mesial Surface of the 
Crown (Fig. 102). — Aside from 
this surface being of less extent 
than the corresponding surface of 
the first molar, there are no other 
differences of importance. In 
many instances, however, there 
is a decided tendency for the 
surface to be concave from 
buccal to lingual, the convex 
distal surface of the first molar 
closely fitting into this concavity. 
Another variation which is fre- 
quently observed is that of the 
longer and more gradual sweep which it takes in passing into the 
lingual surface. 

The Distal Surface of the Crown (Fig. 103).— This differs from 
the distal surface of the first molar principally in its more pronounced 



Lingual 
Root 



Cervical 
Line 



Mesiolin- 
gual Angle 




Mesiobuc- 
cal Root 



Fig. 102. 



Mesiobuc- 
cal Angle 



-Upper Second Molar, Mesial 
Surface. 



164 



ANATOMY. 



Distobuc- 
cal Root 



Mesiobuc- 
cal Root 



Lingual 
Root 



Distoling- 
ual Groove 



convexity. Its general form is also much influenced by the nature of 
the two distal cusps. If one or the other of these is sparingly developed, 
either the buccal or lingual half of the surface, as the case may be, is 
quickly rounded off to pass into the deficient lobe. 

The Angles of the Crown. — The increased inclination for the 
crown of this tooth to general convexity dispels, in a measure, the presence 
of angles, as such, in correspondence with the four corners of the first 
molar. In some instances the crown is represented as a fairly well- 
formed quadrilateral, in which case the angles are well defined, but 

usually this outline is so much 
broken by a mesiodistal com- 
pression that the angular form 
of the crown is entirely abolished. 
But, whatever the form of the 
crown may be, it is well to ad- 
here to the commonly accepted 
term, and speak of that point at 
which the sides of the crown 
unite as the angles, each being 
named in accordance with its 
location. 

The Neck of the Tooth.— 
The principal variation between 
the neck of this tooth and that 
of the first molar, is that produced by the greater general convexity of 
the crown, which contributes to ; the production of a neck much more 
constricted. There is also a greater variety in the contour of the neck, 
incident to the variation in the general outline of the crown. 

The Roots of the Upper Second Molar. — These are the same 
in name and number as those of the first molar — two buccal and one 
lingual. In many respects they differ from the roots of the first molar. 
They are much smaller, frequently inclined to cluster together, and are 
often fused, in some instances, all three being united, in others the union 
existing between but two. When isolated, each root usually presents a 
decided distal curve near its apical third. When the crown is flattened 
from mesial to distal, as before described, the distobuccal root is forced 
to occupy a position much more to the lingual than that assumed by 
the mesiobuccal root. The lingual groove seldom passes over the lingual 
root, as observed on the first molar. 




Fig. 103. 



-Upper Second Molar, Distal 
Surface. 



UPPER THIRD MOLAR. 165 



UPPER THIRD MOLAR. 




10th year nth year 12th year 14th year 1 8th year 

Fig. 104. 

Calcification Begins, Ninth Year. 

Calcification Completed, Eighteenth to Twentieth Year. 
Erupted, Seventeenth to Twentieth Year. 
Average Length of Crown, .24. 

Average Length of Root, .44. 

Average Length over All, .68. 

Calcification of this tooth takes place in precisely the same manner 
as that in the first and second molar, with the exception of the number 
of lobes, which are sometimes three and sometimes four. The lime- 
salts begin to accumulate between the eighth and ninth year, and con- 
tinue with somewhat more activity than that of the first and second molar. 
Between the ninth and tenth year the three or four cusps, of which the 
future tooth is to be composed, have coalesced, and by the eleventh year 
calcification in the crown of the tooth is completed; at the end of the 
following year the roots, which are variable in number, have made 
considerable progress; at the fourteenth year they are calcified to about 
half their length, while at a period between the eighteenth and nineteenth 
year the formative process is completed (Fig. 104). This tooth, like 
the cuspid, is usually fully formed before eruption takes place. 

This tooth is subject to a greater variety of form than any other; 
in rare instances it is similar in general outline and cusp formation to 
the first molar, but in a vast majority of cases it is dissimilar, the most 
constant deviation being its size, which on the average is about one- 
third less. In the accompanying illustration (Fig. 105) the forms most 
frequently met with are shown. It will be observed that the contour 
of the tooth in general is much more rounded than either the first or second 
molar. The buccal angles of the crown are alone well marked, the 



1 66 



ANATOMY. 



mesial and distal surfaces passing into the lingual surface by a long, 
gradual sweep, and thus obliterating the lingual angles. In many 
instances the tooth is tritubercular, and is usually made so by the absence 
or diminutive size of the distolingual cusp. Just as this cusp was inclined 
to degenerate in the second molar, so we find this retrograde develop- 
mental tendency increased in the third molar. With this change in the 
construction of the occlusal surface, there is a corresponding variation 
in the grooves, ridges, and fossae. 




Fig. 105. — Various Types of Upper Third Molar. 



Mesial Surface of the Crown (Fig. 106). — In many particulars 
this surface corresponds in form and outline to the mesial surface of the 
first molar; it is, however, usually much more convex, seldom presenting 
a concavity or even a positive flatness. The surface is not only rounded 
from buccal to lingual, but also from the cervical line to its occlusal 
margin. Thus formed, a point of contact is provided near the center 
of the surface. The occlusal margin, the buccal margin, and the cervical 
margin are almost identical to those of the first molar, but in most in- 
stances the lingual margin is wanting, the surface gradually passing 
into the lingual without a decided line of demarcation. 

Distal Surface of the Crown (Fig. 107). — This surface is much 
less extensive in comparison to the size of the crown than the correspond- 
ing surface of either the first or second molars. It is decidedly rounded 



UPPER THIRD MOLAR. 



167 



Buccal 
Boot 

Mesiobuc- 
cal Cusp 




Lingual 
Root 



Mesial Groove 
Fig. 106. — Upper Third Molar, Mesial 
Surface. 



in every direction and is frequently crossed by the distal developmental 
groove, and sometimes by one or more supplemental grooves. The 
general form of the surface is much influenced by the presence or absence 
of the distolingual cusp; with the 
former, the surface is more exten- 
sive, presenting less convexity and 
resembling more closely the distal 
surface of the first and second 
molars; with the latter, the extent Mesiobuc 

' cal Root 

of the surface is decreased and 
the convexity increased. 

Buccal Surface of the 
Crown (Fig. 108). — The mesial 
portion of this surface is in no 
way at variance with the mesial 
portion of the buccal surface of 
the first or second molar, but 
much variety of form exists in 

the distal portion. The buccal groove which serves to separate these 
two portions is located well toward the distal third of the surface, thus 
reducing the size of the distal portion to about one-third that of the 
mesial portion. In general, the surface is but little more convex than 

the corresponding surface of the 
first and second molar. Its mesial 
border is definitely outlined, as 
are also the cervical and occlusal 
margins, but the distal margin 
cannot be definitely located, the 
surface tending to pass gradually 
into the distal surface. Like the 
distal, the extent of this surface 
is much regulated by the size and 
shape of the distobuccal cusp, 
which, like the distolingual cusp, 
is inclined to degenerate. 
Lingual Surface of the Crown (Fig. 109). — Like the distal surface 
previously described, the form of this surface is much influenced by the 
presence or absence of the distolingual cusp. When this cusp is wanting 
or but little developed, the surface presented is decidedly convex and 
smooth; in many instances the mesiodistal curvature described is almost 



Distobuccal 
Root 



Buccal 
Groove 




Lingual 
Root 



Distomar- 
ginal Ridge 

Mesiolin- 
gual Cusp 



Fig. 107. — Upper Third Molar, Distal 
Surface. 



1 68 



ANATOMY. 



Mesiobuc- 
cal Root 



Mesiobuc- 
cal Cusp 




Distobuccal 
Root 



Distal 
Groove 

Distobuccal 
Cusp 



Fig 108 



-Upper Third Molar, Buccal 

Surface. 



a perfect semicircle, and in passing from the cervical line to the occlusal 
margin the surface is carried well toward the center of the crown by 
a long gradual sweep toward the lingual. The lingual groove is usually 
absent. The change in form produced ,by the presence of the disto- 

lingual cusp is principally 
noticeable in a less pro- 
LinguaiRoot nounced convexity and the 
presence of the lingual 
groove, which may be 
noticed as a slight depres- 
sion or as a well-defined 
groove. This groove, when 
present, is always located 
near what would represent 
the distolingual angle of the 
crown; the distolingual cusp 
seldom if ever being of suf- 
ficient size to force its location near the center of the surface as in the 
first molar. In some instances this groove is shown upon the lingual 
surface when the cusp is not present; in this case the distomarginal 
ridge represents in a manner the cusp by its bold, heavy development. 

Occlusal Surface of the Crown (Fig. no).— When looking directly 
upon this surface, an opportunity is presented 
to study the general contour of the crown; the 
most noticeable difference in this respect be- 
tween this tooth and the first molar being ob- 
served in its smaller size, and the absence of 
well-marked angles. It will be noted that the 
mesial and buccal outlines in a measure resem- 
ble the corresponding outlines of the first and 
second molars, but there is scarcely any similarity 
existing when comparing the distal and lingual 
outlines. In some instances the crown is tri 
angular (Fig. 105) ; in others the mesiobuccal and 
bucco-marginal outlines form an obtuse angle, 
the free ends of which are joined together by a 
long semicircle, the latter constituting the distal and lingual outlines (Fig. 
105). Again, almost the reverse of this last-mentioned form is seen, the 
buccal and distal outlines constructing the angle, while the semicircular 
connection between the two is made up of the mesial and lingual outlines. 




Fig. 109.— 
Third Molar, 
Surfac e. 



U p p e r 
Lingual 



UPPER THIRD MOLAR. 



169 



Buccal Buccomar- 
firnnve ginal Ridee 



Distomar- 
ginal Ridge 
Distal 
Fossa 
i)istolin- 
gual Cusp 



The Marginal Ridges. — The me siomar ginal ridge is usually well 
defined, and in most instances is crossed near its center by the mesial 
groove, and frequently by two or more supplemental grooves. This 
marginal ridge is probably the most constant in form, the numerous 
variations to which the surface is liable seldom making any material 
alteration in it. Unlike the ridge above described, the distomar ginal 
ridge is most variable in its construction, nearly all of the forms charac- 
teristic of the occlusal surface exerting a controlling influence over it. 
In rare instances the ridge resembles that of the first and second molars, 
but this form is most frequently interfered with by the absence or diminu- 
tive size of the distolingual cusp, the ridge itself frequently supplying the 
place of the cusp. In many 
cases the ridge is elevated near 
its central part by being rein- 
forced by a portion of the 
oblique ridge. When the dis- 
tolingual cusp is wanting, this 
ridge not infrequently de- 
scends from the summit of 
the distobuccal cusp to the 
distal groove and from this 
point ascends obliquely to the 
summit of the lingual cusp. 
The bucco-mar ginal ridge may be described as similar in most respects to 
the corresponding margin on the first and second molars, the principal vari- 
ation being in the distal half, which is much shorter and less pronounced. 
In the linguomar ginal ridge, again, much variety in outline is noticeable. 
In nearly all instances the ridge is thrown much nearer the center of the 
body of the crown, and, when the tooth is bicuspid in form, it simply 
makes a mesial ascent of the lingual cusp, followed by a gradual incline, 
and passes into the distal ridge, as .above noted. When the distolingual 
cusp is present, the ridge is similar to that upon the first and second 
molars, with the exception of the distal portion, which is less clearly 
marked. 

The Cusps (Fig. no). — As previously stated, the form most frequently 
met with is tritubercular, two of the cusps being upon the buccal and 
one upon the lingual half of the surface. 

The Mesiobuccal Cusp (Fig. no). — This cusp corresponds in nearly 
every particular to the mesiobuccal cusp of the first and second molars; 
it is the most constant in size and form of the three. Its summit is usually 



Mesiobuc- 
cal Cusp 
Central 
Fossa 

Mesiomar- 
ginal Ridge 



Fig. 




-Upper Third Molar, Occlusal 
Surface. 




1 70 ANATOMY. 

angular, and the numerous ridges which descend from it are well defined 
and similar in name and number to those of the first molar. 

DistobuccalCusp (Fig. no). — The constant inclination to degeneracy 
in the distal portion of the crown of the tooth is noticeable in this cusp, 
which is much smaller than the mesiobuccal and scarcely half as large 
as the corresponding cusp of the first and second molars. In some 
instances, however, it is inferior only in size, retaining its angularity, 
being possessed of small but well-defined ridges. 

The Lingual Cusp (Fig. no). — When the three cusps alone are 
present, this one is much the largest, the extent of the surface covered 

being all of the lingual half of the crown. The 
summit of the cusp, which is thrown well toward 
the center of the body of the crown, is prominent, 
but seldom angular. Only in rare instances will 
there be found a lingual ridge descending there- 
from, but the central incline is usually marked 
by a number of wrinkles or folds of enamel re- 
sembling minute ridges. The central boundary 
of this cusp is marked by the mesial and distal 
Fig. in.— Upper Third developmental grooves. 

Molar, Occlusal Surface, _, . .. . _, ._. . T . it 

with Distoiingual Cusp The Distolingual Cusp (Fig. no).— It is the 

and Distal Fossa Poorly p resence or absence of this cusp that contributes 

most to the variations present in the crown. 
When present, it is usually diminutive in size, and is without definite 
form. In many instances nature is apparently attempting to cast it off 
in precisely the same manner in which she is attempting to add to the 
first molar by a development of the "fifth cusp," the distobuccal cusp 
appearing to hang to the distolingual angle of the crown in a manner 
very similar to the "fifth cusp." W r hen thus situated, it is separated 
from the body of the crown by a groove, which cannot be considered as 
being upon the occlusal surface. When located in its normal position, 
it has for its inner boundary the distolingual groove. 

The Fossae and Grooves of the Occlusal Surface. — The great 
variety and form common to this surface exerts a controlling influence 
over the size, number, and position of the grooves and fossae. In the 
tritubercular class the central fossa alone is present. The developmental 
grooves, with the exception of the buccal, are not definitely outlined, 
but, descending toward the fossae from the central incline, are numerous 
small ridges divided from each other by a like number of diminutive 
supplemental grooves. The distal groove is sometimes well defined, 



UPPER THIRD MOLAR. 171 

and crosses over the oblique ridge, which in this type becomes the disto- 
marginal ridge. When the distolingual cusp is present, all of the ridges 
and grooves are more pronounced. In this case the central fossa cor- 
responds more closely to the central fossa of the other molar teeth, this 
resemblance increasing just in proportion as the size of the distolingual 
cusp increases. The distal fossa, in a vast majority of instances, is 
present as a mere pit; the size of this fossa is likewise much controlled 
by the extent of development in the distolingual cusp. Where the disto- 
marginal ridge is supplementary to the distolingual cusp, the distolingual 
groove lies between the former and the oblique ridge. Another peculi- 
arity found only upon the occlusal surface of this tooth is, what appears 
to be an effort upon the part of the cusps to cluster toward the center. 
This is common only to those teeth possessing three cusps, and accompany- 
ing this form the central fossa shows a number of fantastically arranged 
grooves and ridges which ascend the cusps, passing over the marginal 
ridges and breaking them into a number of small tubercles. 

Temperamental Types. — The third molar tooth is probably less 
influenced by the character and habits of the individual than any other 
tooth in the mouth. The inclination to a general degeneracy is no doubt 
favored by civilization. With a constant decline in the functional 
activity, brought about by the present culinary methods common to 
civilization, this tooth in a measure becomes useless, and nature is 
gradually making an effort to cast it off. While there are undoubtedly 
many individuals possessed of the highest mental attainments with the 
third molar as fully developed as either the first or the second, this con- 
dition is usually confined to those possessed of little intellectuality. If, 
in general, the temperament of the subject be taken into consideration, 
the cusp-formation on this tooth will correspond in a relative degree 
to that on the bicuspids and molars. 



CHAPTER IX. 

A Description of the Lower Teeth in Detail. — Calcification, Erup- 
tion and Average Measurements. — Their Surfaces, Ridges, 
Fossae, Grooves, Sulci, etc. 

THE LOWER TEETH. 

In most respects the anatomy of the lower teeth is similar to that 
of the upper, but in each class we find a slight variation existing between 
the two sets. As compared to the upper incisors, the crowns of the 
lower incisors are more slender and somewhat more angular in outline. 
The roots are more slender, proportionately longer, more flattened 
laterally, and seldom crooked. The crowns of the lower incisors 
are probably more constant in form than those of any other teeth, 
seldom varying except in size. The mesiodistal measurement of the 
crown of the lateral incisor is a trifle greater than that of the central, a 
condition exactly the reverse to that of the upper incisors. The labial 
and the lingual surfaces of these teeth are smooth, and, with the exception 
of young teeth, show but little trace of the developmental process by 
the presence of grooves, fissures, etc. The outline of the lower cuspids 
is almost identical to that of the corresponding teeth in the superior arch, 
excepting that they are in every way more slender. The bicuspids are 
proportionately smaller in every direction than those of the upper jaw, 
their cusps are smaller, and they are seldom found with more than one 
root. The crowns of the lower molars are somewhat larger than those 
of the upper, and are provided with five cusps instead of four,* and they 
are attached to the alveolus with two, instead of three, roots. In the 
incisors, cuspids, and bicuspids the process of development is the same 
as in the corresponding upper teeth, calcification taking place from the 
same number of centers along the coronal extremities. In the molars, 
however, development may proceed from five centers instead of four, 
as in the upper molars. The manner of development, and the period 
at which this action takes place, so nearly corresponds with that of the 
upper teeth that the process will not be repeated. 

*The lower first molar has five cusps in ninety per cent, of cases, while in the second 
five cusps are present in about fifty per cent. 

I 7 2 



LOWER CENTRAL INCISOR. 



*73 



LOWER CENTRAL INCISOR. 



Labial 
Grooves 



Distal 
Angle 



Calcification Begins, First Year after Birth. 

Calcification Completed, about the Tenth Year. 
Erupted, Seventh to Eighth Year. 
Average Length of Crown, .34. 

Average Length of Root, .47. 

Average Length over All, .81. 

Like the upper central incisor, this tooth presents for examination 
four surfaces, a cutting-edge, and various angles, margins, etc. By the 
union of the labial and lingual surfaces at the cutting-edge the incisive 
feature is established and the double incline plane common to incisors 
produced. 

The Labial Surface of the Crown (Fig. 112). — This surface is 
smooth and convex, its general outline resembling an inverted cone, 
the base of which is formed by the cutting-edge and the apex by the 
cervical line. The margins of the surface 
are, with the exception of the cutting-edge, 
not so well defined as those of the upper 
central. Near the cutting-edge the mesial 
and distal margins pass somewhat abruptly 
into the respective lateral surfaces, but as 
the neck of the tooth is approached they are 
much rounded. The incisive marg" 
squarely cut, and is nearly at right angles 
with the long axis of the tooth. The cervical 
margin is fairly well defined, and is deeply 
concave in the direction of the root. Except 
in very young teeth, this surface is seldom 
much broken by the labial grooves; but in 
certain types one or more transverse ridges 
may be found occupying the cervical third. 
The mesiodistal diameter at the cutting- 
edge is about one-third greater than at the cervical line, and, while 
these measurements are likely to vary in accordance with the tempera- 
ment of the subject, this variation is not so pronounced as in the upper 
incisor. The surface is frequently inclined to flatness near the incisive 
margin, the general convexity becoming more marked as the cervical 
line is approached. 

The Lingual Surface of the Crown (Fig. 113). — In general out- 
line this surface resembles the labial, with the exception of the cervical 



IS Cervical 
Line 




Mesial 
Angle 



Cervica 
Ridge 



Fig. 112. — Lower Incisor, Right 
Side, Labial Surface. 



174 



ANATOMY. 



Lingual Grooves 



Distomar- 
ginal Ridge 

Cervical 
Ridge 




Mesiomar- 
ginal Ridge 
Lingual 
Fossa 



Fig. 113. — Lower Incisor, Right Side, 
Lingual Surface, Strongly Developed. 



margin, the lines of which are somewhat more acute. The surface 
presents a marked concavity from the cutting-edge to the cervical ridge, 
and also a slight transverse concavity near the incisive margin. All of 

the margins are more definite than 
those of the labial surface. The 
mesial and distal margins are formed 
by the marginal ridges common to 
these borders, but these ridges are not 
so well defined as those of the upper 
incisors. The cervicomarginal ridge 
is present as a well-rounded band of 
enamel, but is never a well-defined 
cingulum, or cuspule. The depres- 
sion between these marginal ridges is 
so slight that it can scarcely be re- 
ferred to as a fossa, although usually 
characterized as the lingual fossa. 
The lingual grooves are generally more pronounced than the corres- 
ponding developmental grooves of the labial surface, but end more 
or less abruptly before reaching the cervical ridge. The mesiodistal 
measurements of the surface are a trifle less than the corresponding 
measurements of the labial surface. cutting-edge 

The Mesial Surface of the Crown (Fig. 
114). — The outline of this surface is exactly 
the reverse of the labial and lingual just de- 
scribed, being a cone, with its base directed 
downward or in the direction of the root, 
while its apex is formed by the mesial ex- 
tremity of the cutting-edge. The cervical 
margin of the surface, or that represented by 
the base of the cone, is concave; the labial 
and lingual margins are rounded over the 
cervical third and inclined to angularity near 
the cutting-edge. There is a slight convexity 
over the entire surface, which is most marked 
near the center. The lingual half of the 
cervical portion slopes away to the distal, 
passing gradually into the lingual surface. By the union of this surface 
with the cutting-edge and the labial and lingual surfaces, the mesial 




Cervical 

Line 
Cervical 

Ridge 



Fig. 114. — Lower 
Incisor, Mesial Sur- 
face. 



angle of the crown is formed. 



This angle is well outlined and reaches 



LOWER CENTRAL INCISOR. 



175 



Lingual 
Ridge 
Cervico- 
lingual 
Ridge 




Labial 
Ridge 
Cervical 
Line 



Fig. 115. — Lower Incisor, Distal 
Surface. 



out toward the median line, giving to this portion of the crown a promi- 
nent appearance. Near the cutting-edge the surface presents a slightly 
rounded prominence, which provides a 
point of contact with the corresponding 
tooth of the opposite side. 

The Distal Surface of the Crown 
(Fig. 115). — In a general way this sur- 
face closely resembles that of the opposite 
or mesial side of the crown. Near the 
cutting-edge the surface is usually more 
prominent and presents a more marked 
convexity, and near the cervical margin 
it is flattened and sometimes slightly con- 
cave. The union of this surface with the 
cutting-edge and the labial and lingual 
surfaces forms the distal angle of the 
crown, which, like the mesial angle, is 

square and well defined. The margins of the surface are in no 
different from those of the mesial surface. 

The Cutting-edge. — In the young tooth this incisive 

is thin and generally divided into three distinct parts 
(Fig. 116) by the developmental grooves, but these 
disappear so early that they can scarcely be considered 
in connection with a description of the fully developed 
tooth; in fact, the cutting-edge of this, as well as that 
of all the lower incisors, is so susceptible to change by 
mechanical abrasion that a normal condition is of but 
short duration. After the disappearance of the 
primitive cusps, and before further abrasion has taken 
place, the edge is fairly sharp and placed nearly at 
right angles with the crown. As in the upper incisors, 
the cutting-edge is in a line with the long axis of the 
tooth. The labial margin of the edge is slightly con 
vex, while the lingual is irregularly concave to the 
same extent. 

The Cervical Margin. — This marginal line, 
which is marked by the extent of the enamel cap, 
corresponds closely to that of the upper incisors, 
dipping down with a graceful concavity on the labial and lingual surfaces 
with a corresponding convexity on the mesial and distal surfaces. 



way 



margin 



Developmental Grooves 




Fig. 116. — Young 
Lower Incisor, with 
Cutting-edge, show- 
ing the Lines of 
Development. 



176 ANATOMY. 

The prominence of this enamel margin, together with the nature of 
the curvature, is much influenced by the tooth type. 

The Neck of the Tooth. — A distinctive feature of this tooth is 
found in the convergence of its mesial and distal surfaces in passing 
from the cutting-edge rootward, thus producing a neck much constricted 
from mesial to distal. When examined from either the mesial or distal 
surface, this feature is scarcely noted, the crown passing into the root 
with little more than the cervical line as a mark of separation. The 
labial and lingual portions of the neck are rounded and narrow, while 
the two lateral sides are flat and broad. 

The Root. — The root of this tooth is usually smaller than that of 
any other tooth in the mouth. It is much flattened from mesial to distal, 
while the labial and lingual aspects are rounded and narrow. Besides 
being flattened and broad, the mesial and distal sides are usually found 
with a longitudinal depression extending from a point near the base of 
the root almost to its apex. These surfaces gradually taper from the 
base to the apex, while the labial and lingual first widen from the base 
and then gradually taper to the apex. The contour of the root-base 
is generally reduced at the apical extremity, although in some instances 
the latter is a rounded point. While the root of the tooth is usually 
straight, there is sometimes a tendency for the apical third to have a 
slight distal inclination. 



w 



LOWER LATERAL INCISOR. 

Calcification- Begixs, First Year after Birth. 

Calcification Completed, Tenth to Eleventh Year. 
Erupts, Eighth to Ninth Year. 
Average Length of Crown, .35. 

Average Length of Root, .50. 

Average Length over All. .85. 

The crown of this tooth differs from the central incisor in being 
broader from mesial to distal at the cutting-edge, resulting in a crown 
more strongly bell-shaped. The cutting-edge, instead of being at right 
angles to the long axis of the tooth, slopes to the distal at the expense 
of the distal angle, which is much rounded, while the mesial angle closely 
resembles the corresponding angle of the central incisors. The labial 
and mesial surfaces do not differ materially from the corresponding 
surfaces of the central incisor, excepting that the lingual more frequently 
shows the lines of development, and the distal is at variance in having 
that portion which contributes to the formation of the distal angle ex- 
tended and prominent. The marginal ridges of the lingual surface are 



LOWER CUSPID. 



I 7 7 



probably more definitely outlined than those of the central incisor, and 
the crown in general presents a stronger appearance. The neck of the 
tooth is similar to the neck of the central incisor, as is also the root, with 
the exception of a slight addition to its length. 



Labial Grooves 



LOWER CUSPID. 

Calcification Begins, Third Year after Birth. 

Calcification Completed, Twelfth to Thirteenth Year. 
Erupts, Twelfth to Thirteenth Year. 
Average Length of Crown, .40. 

Average Length of Root, .60. 

Average Length over All, i.oo. 

There is probably a greater similarity existing between the upper 
and lower cuspid teeth than in any other class of teeth in the mouth. 
Occupying as they do a prominent position in the dental arch, and being 
called upon to perform the double function of incising and tearing the 
food, their crowns are strong and heavy-set, and their roots long and 
firmly anchored in the alveoli. Like the upper cuspid, the crown of the 
lower is surmounted by a single 
cusp, from the summit of which 
descend a mesial and a distal 
cutting-edge. There is also a 
labial, lingual, mesial, and distal 
surface presented for examination. 

The Labial Surface of the 
Crown (Fig. 117). — The crown of 
the tooth, being a little longer than 
that of the upper cuspid, gives to 
this surface the appearance of be- 
ing more slender, when in reality 
there is but little difference in the 
width of the two teeth. This sur- 
face is smooth and convex, and, 
while the labial grooves are usually 



Mesial Cut- 
ting-edge 
Mesia 
Argle 
Labia 
Ridge 
Cervica- 
Ridge 




Distal Cut- 
ting-edge 

Distal Mar- 
gin 



Fig. 117. — Lower Cuspid, Labial Surface. 



present, they are not so marked as those found upon the corresponding 
upper tooth. A pronounced feature of the surface is the labial ridge, 
which extends from the summit of the cusp to the cervical line, provid- 
ing additional strength to the crown. Aside from this ridge and the labial 
grooves, the surface is occasionally broken by one or more transverse 
ridges over the cervical portion. The margins of the surface closely 
resemble those of the upper cuspid, the incisive and mesial being definite 



i 7 8 



ANATOMY. 



Lingual Grooves 



Distal Angle 

Distomar- 
ginal Ridge 




Fig. 118. — Lower Cuspid, Lingual 
Surface 



in character, while the distal is made equally indefinite by the passing 
of the labial into the distal surface by a gentle curve. 

The Lingual Surface of the 

Crown (Fig. 118). — The ridges and 

grooves of this surface are far less 

bold in their character than those 

Ridge a of the upper cuspid. The lingual 

Mesiomar- . , , . , 1 • • j i r 

ginai Ridge ridge, which divides the surface 

Cervical 

Ridge into two equal parts, extends from 
the summit of the cusp to the base 
of the cervical ridge, while the 
marginal ridges pass rootward from 
the angles of the crown, and, unit- 
ing, form the cervical ridge. The 
slight depressions between the 
lingual ridge and the marginal 
ridges correspond to the palatal 
grooves of the upper cuspid, but 
in this tooth partake more of the 

nature of fossae. 

The Mesial Surface of the Crown (Fig. 119). — A peculiarity 

found in connection with this surface is the general plane existing between 

the crown and root-surface. In all 

other teeth the mesial and distal 

surfaces are found to bulge some- 
what beyond the corresponding 

surface of the root, but this surface 

of the lower cuspid is not only 

usually in a direct line with the 

mesial surface of the root, but is 

occasionally inclined to the distal, 

resulting in a crooked or bent ap- 
pearance to the tooth. In addition 

to this individual peculiarity, the 

surface is flat and passes by a long 

curve to meet the lingual surface. 
The Distal Surface of the 

Crown (Fig. 120). — This surface is 

somewhat less in extent than the mesial surface, and, in place of being 

flat and in line with the root-surface, that portion near the angle of 



Labial 
Ridge 



Cervical 
Ridge. 




Lingual 
Ridge 

Mesiomar- 
ginal Ridge 



Cervical Line 



Fig. 119. 



-Lower Cuspid, Mesial 
Surface. 



LOWER CUSPID. 



I 79 



Lingual 
Ridge 



Cervicolin- 
gual Ridge 



Mesial 
Groove 



the crown presents a marked convexity, while that near the cervical line 
is frequently slightly concave. This general form of the surface further 
assists in producing the distal crook previously referred to. The lingual 
margin is well defined and somewhat angular, while the surface passes 
so gradually into the labial that a positive line of demarcation can scarcely 
be said to exist. 

The Cusp and Cutting-edges. — In most respects these are similar 
to the corresponding parts of the upper cuspid. The length of the 
mesial cutting-edge is usually somewhat less than that of the distal, but 
this difference is seldom so marked 
as that found in the upper cuspid. 
The mesial and distal angles of the 
crown are equally as pronounced 
as those of the corresponding upper 
tooth. 

The Neck of the Tooth.— 
This is shown by a fairly well- 
marked constriction, but the pass- 
ing of the mesial surface of the 
crown into the mesial surface of 
the root is not broken by this cir- 
cular depression. On account of 
this latter feature the neck of this 
tooth is somewhat less pronounced 
than that of the upper. 

The Root of the Lower Cuspid. — The root of this tooth is some- 
what shorter and more flattened on its mesial and distal sides than that 
of the upper cuspid, this lateral flatness frequently amounting to a 
decided longitudinal depression or groove. As referred to in the descrip- 
tion of the crown, the mesial side of the root is continuous in a direct 
line with this surface of the crown, but as the apical third of the root is 
approached, there is frequently found a slight distal inclination which 
affects alike both the mesial and distal sides. The labial and lingual 
surfaces of the root are abruptly convex and taper very gradually from 
the cervical line to the apex, while the mesial and distal surfaces taper 
much more rapidly, the four ending in a slender apex usually flattened 
from mesial to distal. 




Fig. 120. 



-Lower Cuspid, Distal 
Surface. 



i8o 



ANATOMY. 



THE LOWER BICUSPIDS. 

In many respects these teeth are similar to the bicuspids of the 
upper jaw, the chief differences being that they are somewhat shorter 
and smaller in every respect. Their crowns are much more rounded and 
the cusps are never so strongly developed. Unlike the upper bicuspids 
the buccal and lingual cusps are connected by a transverse ridge. The 
roots are much less flattened from mesial to distal, and are seldom 
bifurcated. 



LOWER FIRST BICUSPID. 

Calcification Begins, about the Fourth Year. 

Calcification Completed, Eleventh to Twelfth Year. 
Erupted, Tenth to Eleventh Year. 
Average Length of Root, .30. 

Average Length of Crown, .54. 

Average Length over All, .84. 

In general, the crown of this tooth is much more rounded and 
smaller in all its measurements than that of the upper bicuspid. The 
buccal surface presents a much greater convexity, which results in 
forcing the summit of the buccal cusp well toward the center of the long 
axis of the tooth. The mesiodistal and buccolingual measurements of 
summit of Buccal Cu the crown are nearly equal, and 

about correspond to the max- 
imum length of the crown. As 
in the upper bicuspids, the de- 
velopment of this tooth is similar 
to that of the incisors and cus- 
pids, the buccal cusp being de- 
rived from three lobes, while 
the lingual results from a single 
center. 

The Occlusal Surface of the Crown (Fig. 121). — This surface 
is so unlike that of the corresponding surface of the upper first bicuspid 
that a separate description without further comparative reference is 
required. In general outline the form of a rounded triangle is approached, 
the buccal margin serving as one side of the triangle, while, by the union 
of the mesial and distal margins to form the lingual, the remaining sides 
are established. 

The Buccal Cusp. — As previously stated, the summit of this cusp 
is thrown well toward the center of the surface. Descending from it 



Buccal Ridge 



Distal Pit 




Buccal Groove 
Mesial Pit 

Triangular 
Ridge 



Lingual Ridge 

FlG. 121. — Lower First Bicuspid, 

Occlusal Surface. 



THE LOWER BICUSPIDS. 



181 



Buccal Ridge 



Cervical 
Ridge 




Bucca 
Groove 



Cervica 
Line 



Fig. 122. — Right Lower First Bi- 
cuspid, Buccal Surface. 



are four well-defined ridges — the buccal ridge to the buccal surface, 

the mesial cutting-edge, the distal cutting-edge, and the triangular 

ridge, the latter descending in a lingual 

direction to meet the lingual ridge or 

cusp. This ridge divides the surface 

into two parts, the center of each being Di ^ b £, ccal 

marked by a well-defined pit — the mesial 

and distal pits. The mesial and distal 

cutting-edges are frequently crossed by 

the buccal grooves, and mark the line 

of union between the central and two 

lateral lobes of the buccal cusp. The 

marginal ridges, one of which begins at 

the mesial angle and the other at the 

distal angle, pass to the lingual, where 

they unite to form the Ungual ridge or 

cusp. 

The Lingual Cusp. — This cusp is 

seldom well developed, and corresponds to the cervical ridge of the 

incisors and cuspids. The extent of development in the lobe is ex- 
tremely variable, in some instances amounting to little more than a 

continuation of the mesio- and disto- 
marginal ridges, while in others there is 
a building-up of the enamel in the form 
of a small tubercle. When this latter 
condition is present, the triangular ridge 
of the buccal cusp contributes to its 
formation. The triangular ridge fre- 
quently divides into two or more 
smaller ridges, which usually end in the 
mesial pit, but in some instances they 
continue to the lingual, and divide the 
lingual ridge into two or more smaller 
tubercles. 

The Buccal Surface of the Crown 
(Fig. 122). — This surface is smooth and 
convex in all directions, and in general 
outline there is but little variation be- 
tween it and the corresponding surface of the upper first bicuspid. 

It is traversed from the point of the cusp to the cervical line with a 



Buccal Cusp Triangular 
Ridge 



Distal Pit 




Mesial Pit 
Lingual 
Ridge 



Fig. 123. — Left Lower First Pi- 
cuspid, Lingual Surface. 



l82 



ANATOMY. 



Buccal Cusp 



Lingual Cusp 
or Ridge 




Fig. 124 — Left Lower First Bicus- 
pid, Mesial Surface. 



rounded ridge, the buccal ridge, upon either side of which are the 
buccal grooves. 

Lingual Surface of the Crown (Fig. 123). — This surface is more 

or less extensive in accordance with 

the character of the lingual lobe. In 

most instances the measurement from 

Buccal the summit of the cusp or ridge to the 

Ridge . to 

cervical line is about one-half that of 
the same measurements on the buccal 
surface. From mesial to distal a well- 
rounded convexity is present, while 
from the occlusal margin to the 
cervical line it is straight or only 
slightly convex. The surface passes 
so gradually into the mesial and distal 
surfaces that no definite lateral mar- 
gins exist. 

The Mesial Surface of the 
Crown (Fig. 124). — In the region of 
the occlusal margin this surface is prominent, with a marked convexity 
from buccal to lingual; but as the cervical margin is approached, the 
surface recedes to the distal, and is 
flattened or is' possessed of a slight 
general convexity. The occlusal and 
cervical margins alone are well de- 
fined, the buccal being gracefully 
rounded, while the surface passes to 
the lingual with a long curve. 

The Distal Surface of the 
Crown (Fig. 125). — There is but 
little difference between this and the 
mesial surface; the occlusal portion of 
the surface is somewhat less promi- 
nent, resulting in less of the bell- 
shaped appearance to this side of the 
crown. 

The Neck of the Tooth.— The 
neck of this tooth is marked by a well-defined constriction, the enamel 
of the crown suddenly folding in to meet the cementum of the root at 
the cervical line, forming a band or ridge which completely encircles the 



Triangular Ridge 



Distal Pit 



Lingual Ridge 




Cervical 
Ridge 



FlG. 125. — Right Lower First Bi- 
cuspid, Distal Surface. 



THE LOWER BICUSPIDS. 



I»3 



tooth. The amount of constriction appears to be evenly distributed 
between the various parts, so that, viewed in all directions, the neck 
becomes a distinctive feature of the tooth. 

The Root of the Lower First Bicuspid. — The root of this tooth 
is usually straight and tapers gradually from base to apex. In rare 
instances it is bifurcated, and when thus formed, those portions beyond 
the point of separation are more or less crooked. In the single root the 
apical third often curves slightly to the distal. The buccal and lingual 
sides are convex throughout their entire length, while the mesial and 
distal may be slightly convex, flattened, or provided with a slight longi- 
tudinal concavity. In passing from buccal to lingual the mesial and 
distal sides converge, thus resulting in a narrowing of the lingual side 
of the root. 



Buccal Cusp 



LOWER SECOND BICUSPID. 

Calcification Begins, Between the Fourth and Fifth Year. 
Calcification Completed, Eleventh to Twelfth Year. 
Erupts, Eleventh to Twelfth Year. 
Average Length of Crown, .31. 

Average Length of Root, .56. 

Average Length over All, .87. 

In general contour this tooth is similar to the lower first bisuspid, 
excepting that the crown is somewhat more rounded and the lingual 
cusp more fully developed, this latter feature causing it to closely resemble 
the upper bicuspids. The crown is frequently a trifle shorter than that 
of the lower first bicuspid, but the length 
of the root generally exceeds that of the 
latter, making this the longer tooth of 
the two. 

The Occlusal Surface of the 
Crown (Fig. 126). — The occlusal sur- 
face of this tooth presents a greater 
variety in form than any other tooth of 
its class. The general outline of the 
surface is that of a broken circle, in 
most instances the mesial and distal 
margins showing almost as much of a 
convexity as that of the buccal and lingual. The summit of the 
buccal cusp usually extends well toward the center of the surface, 
but it is sometimes forced toward the buccal by an increased develop- 
ment in the lingual cusp. The buccal grooves, which cross the mesial 



Triangular 
Ridge 



Distal Pit 



Buccal 
Groove 




^usp 



Fig. 126 



Lingual Ci: 
— Lower Second Bicuspid, 
Occlusal Surface. 



1 84 



ANATOMY. 



Buccal Ridge 



Distobuccal 
Angle 
Cervical 
Ridge 




Buccal 
Groove 



Cervical 
Line 



FlG. 127.- 
cuspu 



and distal cutting-edges of the buccal cusp, are seldom so well defined 
as those of the first bicuspid, but they occasionally pass over these 

marginal ridges and form well-marked 
grooves, which end in the mesial and 
distal pits. The triangular ridge of the 
buccal cusp is usually more prominent 
than in the first bicuspid, and divides 
the surface into two portions, which are 
about equal in extent, the center of each 
portion being provided with a small pit 
— the mesial and distal pits. As in the 
first bicuspid, the mesio- and disto- 
marginal ridges begin at the mesial and 
distal angles of the crown, pass to the 
lingual, and, uniting, form the lingual 
ridge or cusp. The lingual cusp, while 

Right Lower Second Hi- ,. ,, , , , . 

(1 Buccal Surface generally well developed, is never so 

prominent as the buccal. The lingual 
lobe is sometimes divided by a groove which passes from buccal to 
lingual, thus forming three cusps upon the surface. When this latter 
condition is present, the mesial and distal grooves are fully outlined 
from the mesio- and disto-marginal Buccal Grooves 

ridges to the center of the surface, where 
they unite with the groove previously 
referred to and form a central pit or 
fossa. Another form frequently met 
with is one in which the surface closely 
resembles that of the upper bicuspids, 
two well-defined cusps being present, 
separated from each other by a central 
groove, which passes from mesial to 
distal and joins the triangular grooves 
at these joints. The resemblance to the 
upper bicuspids is further increased by 
the presence of two small pits, one on 
the mesial and one on the distal half of 
the surface. 

The Buccal Surface of the Crown 



Distomar- 
ginal Ridge 




Summit 
of Lin- 
gual Cusp 



Fig 128. — Lower Second Bicuspid, 
Lingual Surface. 

(Fig. 127). — The principal 



variation between this and the buccal surface of the lower first bicuspid 
is that it is less extensive and the buccal grooves somewhat less defined. 



THE LOWER BICUSPIDS. 



185 



Lingual Cusp 



Mesiomar- 
ginal Ridge 



Triangular Ridge 




Buccal Cusp 
Buccal Ridge 



Fig. 129. 



—Lower Second Bicuspid, 
Mesial Surface. 



It presents a general convexity, which is most pronounced near the 
center, between which point and the occlusal margins it is slightly inclined 
to flatness. The summit of the buccal cusp is usually to the mesial of 
the center of the occlusal mar- 
gin, so that the mesial cutting- 
edge is considerably longer than 
the distal, this fact also resulting 
in forcing the buccal ridge to the 
mesial of the center of the sur- 
face. The mesial angle of the 
crown, as observed when looking 
directly upon the buccal surface, 
is in a direct line with the mesial 
side of the root, while the distal 
angle extends beyond this cor- 
responding line, and gives a 
prominent or bulging appearance 
to this section of the crown. 

The Lingual Surface of the Crown (Fig. 128).— Proportionately, 
this surface is more extensive than the corresponding surface of the first 
bicuspid, this increase being produced by the additional development 
of the lingual cusp. It is well rounded from mesial to distal, and passes 

into these surfaces without the existence 
of a positive line of separation. From 
the cervical line to the occlusal margin a 
slight convexity is present. The general 
outline of the surface is much influenced 
by the conditions present upon the oc- 
clusal surface. 

The Mesial Surface of the Crown 
(Fig. 129). — In the region of the occlusal 
margins this surface is decidedly convex 
from buccal to lingual, but in passing 
toward the cervical line a gradual flat- 
ness is apparent, which, however, seldom 
amounts to a perfect plane. While 
there is a gradual convergence of this 
and the distal surface toward the root, it is not so marked as that of 
the first bicuspid, resulting in less of the bell-shaped appearance to 
the crown. 



Buccal Groove 



Lingual 
Cusp 




Buccal 
Cusp 



FlG. 130. 



-Lower Second Bicuspid, 
Distal Surface. 



1 86 ANATOMY. 

The Distal Surface of the Crown (Fig. 130). — The description 
given of the mesial surface applies equally well to this, there being but 
slight variation existing between the two. Occasionally this surface 
will present a greater convexity in the region of the occlusal margin, but 
this is not a constant feature. 

The Neck of the Tooth. — The crown of the tooth being some 
what smaller, and the root proportionately larger and longer than that 
of the first bicuspid, results in diminishing the amount of constriction 
at the neck, and for that reason this feature is less definite. 

The Root of the Tooth. — As previously stated, the root of this 
tooth is larger and longer than that of the first bicuspid. The mesial 
and distal sides are flattened and frequently provided with a longitudinal 
groove. In some instances it is rather blunt, ending in a heavy, rounded 
apex; in others it tapers very gradually from the base to the apex, ending 
in a slim, pointed extremity. 

THE LOWER MOLARS. 

THE LOWER FIRST MOLAR. 

Calcification Begins, about One Month before Birth. 
Calcification Completed, Ninth ro Tenth Year. 
Erupts, Sixth to Seventh Year. 
Average Length of Crown, .30. 

Average Length of Root, .52. 

Average Length over All, .82. 

The process of development in this tooth corresponds to that of 
the upper first molar, calcification beginning upon the various cusps 
as early as the eighth fetal month, the crown being completely calcified 
by the fifth year, the roots formed and the root apices established by 
the eleventh year. There is, however, one important difference between 
the development of this tooth and the corresponding upper tooth : that 
of calcification taking place usually horn five centers instead of four, and, 
as a result, we find the occlusal surface provided with five well-developed 
cusps separated from one another by five developmental grooves. When 
compared with the upper first molar, the crown of this tooth is found to 
be somewhat less in size; in general outline it is subject to a greater varia- 
tion and is much more angular in its nature. The mediodistal measure- 
ment of the crown is nearly always greater than the buccolingual, and 
the length of the crown from the occlusal margins to the cervical line is 
proportionately less than that of the corresponding upper molar. 

The Occlusal Surface of the Crown (Fig. 131). — The general 
outline of the crown is best studied when looking directly upon this 



THE LOWER MOLARS. 187 

surface. Two principal varieties exist: one in which the sides or mar- 
gins of the surface appear to be flattened or straightened out, and the 
other when these same margins are gracefully rounded. In either 
form the buccal line is the longest, so that the mesial and distal lines 
converge to meet the lingual. This common form gives to the buccal 
angles an acute character, while the lingual angles are about equally 
obtuse. The surface is divided into five distinct or developmental 
portions, each of which is surmounted by a cusp, named, as their location 
indicates, mesiobuccal, buccal, distobuccal, mesiolingual, and disto- 
lingual. Separating these parts are Bu c } Bucc . 

five developmental grooves — the Cusp Groove 

mesial, the distal, the buccal, the 






p Mesiobuc 

lingual, and the distobuccal. The D c ^cu"p 8 i calCusp 

four former cross the marginal Distal § I central 

ridges from the various surfaces Gr "" vt ■ | 

B J ■ Mesial 

and end in the central fossa, while Distal Pit j | Groove 

the latter passes from the disto- 
buccal angle and joins the distal 

Disto- Lingual Mesio- 

groove, their union being marked Ungual Groove Ungual 

° Cusp Cusp 

by a slight depression Or pit — the Fig. 131.— Lower First Molar, Occlusal 

distal pit. Branching off from the Surface. 

various grooves are a number of supplemental grooves, the presence of 

which results in the production of a number of smaller ridges. 

The Marginal Ridges of the Occlusal Surface. — Properly speaking 
these are only two in number, the mesiomarginal ridge and the disto- 
marginal ridge. Those margins which correspond to the buccal and 
lingual ridges of the upper molars are so broken by the various cusps 
and developmental grooves that a definite marginal ridge scarcely exists, 
as will be observed by the description of these parts. 

The mesiomarginal ridge is strongly outlined, passing from the 
mesiobuccal to the mesiolingual angle of the crown in the form of a bold 
angular ridge. In some instances it is broken near the center by the 
mesial groove passing over it to reach the mesial surface, in others being 
further divided by numerous small supplemental grooves. 

The distomarginal ridge is much shorter and less decided than the 
mesial, and extends from the distobuccal to the distolingual angle. 
In nearly every instance it is broken by the distal groove, which crosses 
it to reach the distal surface. 

The buccomarginal ridge is formed by the various ridges which 
descend in a mesial or distal direction from the three buccal cusps. 



1 88 ANATOMY. 

Near the center the margin is broken by the buccal groove, and it is again 
broken at its distal third by the distobuccal groove, both of which pass 
over it to reach the buccal surface of the crown. This is much the 
longest margin of the surface, and in its entirety presents a gradual 
buccal convexity. 

The linguomarginal ridge is principally made up of the distal incline 
from the mesial cusp, and by the mesial incline from the distal cusp. 
Near the center it is broken by the lingual groove, which passes over it 
to reach the lingual surface. This margin, unlike the buccal, is not 
always convex, but in many instances is almost a straight line, extending 
from the mesial to the distal angle. 

The Cusps (Fig. 131). — The Mesiobuccal Cusp (Fig. 131). — This 
is usually the largest, though not always the longest, cusp of the group. 
It is bounded by the mesial and buccal surfaces and by the mesial and 
buccal grooves, which together form the mesiobuccal triangular groove. 
Descending from the summit of this cusp to the distal is a well-defined 
ridge — a part of the buccomarginal ridge — while in a mesial and lingual 
direction the descending ridges contribute to both the bucco- and mesio- 
marginal ridges. Descending toward the center of the surface and 
ending in the central fossa is the mesiobuccal triangular ridge. 

The Buccal Cusp (Fig. 131). — This cusp, which is placed a little 
to the distal of the center of the buccal surface, is separated from the 
mesiobuccal cusp by the buccal groove, and from the distobuccal cusp 
by the distobuccal groove. It is about one-half the size of the mesio- 
buccal cusp, and a trifle less in length. Descending from it are two ridges, 
one in a mesial and one in a distal direction, which form a portion of the 
buccomarginal ridge; descending to the buccal surface is the buccal 
ridge, while the central incline gives place to a fourth ridge — the bucco- 
triangular ridge. 

The Distobuccal Cusp (Fig. 131). — This cusp is much the smallest 
of the five, and is located at the distobuccal portion of the crown, in 
some instances being nearest the buccal surface, in others forced to the 
distal by an increase in the size of the buccal cusp. It is separated from 
the buccal cusp by the distobuccal groove, and from the distolingual cusp 
by the distal groove. The ridges which descend from it contribute to 
both the bucco- and disto-marginal ridges, and descending toward the 
distal pit is the distobuccal triangular ridge. 

The Mesiolingual Cusp (Fig. 131). — This cusp is second in size, 
and frequently the longest and most pointed. It has for its boundaries 
the mesial and lingual surfaces, and the mesial and lingual grooves. 



THE LOWER MOLARS. 



189 



The ridge which descends from it in a mesiobuccal direction assists in 
forming the mesiomarginal ridge, while that which passes to the distal 
forms a part of the linguomarginal ridge. In the direction of the central 
fossa a pronounced ridge is present — the mesiolingual triangular ridge 
— which is often supplemented by one or more smaller ridges running in 
the same direction. 

The Distolingual Cusp (Fig. 131). — This cusp usually occupies the 
distolingual portion of the crown, although sometimes being forced well 
toward the lingual by the 



Buccal Groove 



Distobuccal 
Cusp 



Buccal Pit 




Mesioling- 
ual Cusp 

Mesiobuc- 
cal Cusp 



Fig. 132 — Lower First Molar, Buccal 
Surface. 



distobuccal cusp. It is 
separated from the mesio- 
lingual cusp by the lingual 
groove, and from the dis- 
tobuccal by the distal 
groove. Two of the ridges 
which descend from it as- 
sist in forming the linguo- 
and disto-marginal ridges, 
while the one which de- 
scends the central incline is 
the distolingual triangular 
ridge. The central incline 
of the mesiobuccal, buccal, 
mesiolingual, and disto- 
lingual cusps contribute to 
the formation of the central fossa, while the buccal, distobuccal, and 
distolingual central inclines assist in forming the distal pit or fossa. 

The Buccal Surface of the Crown (Fig. 132). — This is the most 
extensive of the lateral surfaces of the crown. It is convex from mesial 
to distal, and also from the occlusal margin to the cervical line. The 
width of the crown from the mesial to the distal angle is always somewhat 
greater than that at the cervical line, the difference being governed by 
the typal form of the tooth. A little to the mesial of the center of the 
surface is the buccal groove, which, after crossing the buccomarginal 
ridge, is usually quite deep; but as it proceeds in the direction of the 
root it gradually disappears, or it may end abruptly in a well-defined pit 
— the buccal pit. The distobuccal groove enters the surface near the 
distobuccal angle, and gradually becomes less pronounced as it passes 
rootward. It is seldom so well defined as the buccal groove, and usually 
ends when about half-way to the cervical line. The occlusal margin 



190 



ANATOMY. 



Distolingual 
Cusp 



Mesiolingual 
Cusp 



Lingual 
Groove 




is made irregular by the presence of the three buccal cusps; the cervical 
margin is nearly straight from mesial to distal, and is surmounted through- 
out by a strong enamel fold, the cervicobuccal ridge. The mesial margin 

is longer than the distal, but neither of 
them is well defined. 

The Lingual Surface of the Crown 
(Fig. 133). — This surface is smooth and 
convex in every direction. It is gener- 
ally divided into two portions, a mesial 
and a distal, which are nearly equal in 
extent. This separation is formed by 
the lingual groove, which is sometimes 
deep and sulcate, at others shallow, and 
not infrequently entirely wanting. The 
surface is nearly one-third less in extent 
than the buccal, the convergence of the 
mesial and distal surfaces in passing to 
the lingual accounting for this difference. 
The occlusal margin is formed by the 
double incline of the two lingual cusps; the cervical margin is either 
straight or slightly concave in the direction of the occlusal surface, 
while the mesial and distal margins are rounded and poorly defined. 

The Mesial Surface of the Crown (Fig. 134). — This surface is 
inclined to flatness, with a slight 
bulging near the center, which marks 
the point of contact with the approxi- 
mate tooth. It is usually smooth, and 
unbroken by developmental or other 
grooves, although the mesial groove 
occasionally traverses it after crossing 
the marginal ridge from the occlusal 
surface. Near the center of the cer- 
vical third a slight concavity is often 
present. The margins of the surface 
are somewhat irregular, the occlusal 



Fig. 



133. — Lower First Molar, 
Lingual Surface. 



Central Fossa 



Mesiobuc- 
cal Cusp 
Cervico- 
buccal 
Ridge 




Mesiolin- 
gual Cusp 



Fig. 



134. — Lower First Molar, Mesial 
Surface. 



margin being made irregularly concave 

by the ridges which descend from the 

two mesial cusps; the cervical margin is slightly concave in the direction of 

the occlusal surface, and, while the buccal margin inclines to the lingual as 

the occlusal surface is approached, the lingual is almost perpendicular. 



THE LOWER MOLARS. 



I 9 I 



The Distal Surface of the Crown (Fig. 135). — Unlike the mesial, 
this surface is possessed of a decided convexity in every direction. It 
is surmounted by a portion of the distobuccal and distolingual cusps, 
and is frequently broken by the distal groove, which reaches it after 
crossing the marginal ridge from the occlusal surface. The occlusal 
margin is irregularly formed of the marginal ridges which descend from 
the distobuccal and distolingual 
cusps; the cervical margin is 
usually straight, while the buccal 



Distobuccal 

and lingual are rounded and in- cusp 




Mesiolin- 
gual Cusp 



Distolin- 
gual Cusp 



Fig. 135. — Lower First Molar, Distal 
Surface. 



definite. 

The Neck of the Tooth.— 

One characteristic feature of 
this tooth is the greater circum- 
ference of the crown at the oc- 
clusal margin over that at the 
cervical line, giving a flaring 
appearance to the crown, and 
resulting in the production of a 
neck which is much constricted. 
This is particularly noticeable 
when looking upon the buccal surface of the tooth; but when looking 
upon the mesial or the distal surface, this feature is not so pronounced, 
although the rather heavy fold of enamel which surmounts the cervical 
line contributes much to the formation of the neck from these aspects. 

The Roots of the Tooth. — The roots of this tooth are two in 
number — one of which is placed beneath the mesial, and the other 
beneath the distal half of the crown — and are named the mesial root and 
the distal root. The fact that the point of bifurcation is constantly in 
close proximity to the neck or crown of the tooth is a sufficient reason 
for the statement that two roots exist, rather than a single root with two 
branches. The roots are both much flattened from mesial to distal, 
and broad at the base from buccal to lingual. 

The mesial root is usually the larger and longer of the two. After 
leaving its base it generally inclines to the mesial, but beyond the center 
of its length it is provided with a distal turn, which in some instances 
amounts to a decided crook. The center of the mesial side is occupied 
by a longitudinal depression, as is also the distal side, making this part 
of the root thin, giving the appearance of an effort to bifurcate, which 
condition is occasionally present. The buccal and lingual sides of the 



192 



ANATOMY. 



root are rounded and smooth, and taper gradually to the apex, which 
is somewhat broadened from buccal to lingual. 

The distal root is usually straight, with a more gradual taper through- 
out, ending in an apical extremity more pointed than that of the mesial 
root. A longitudinal depression is also present upon both the mesial 
and distal sides, but is never so pronounced as that upon the mesial root. 
The buccal and lingual sides are convex and smooth. The root possesses 
little or no inclination to bifurcate. 



Central 
Fossa 



Linguo- 
marginal 

Ridge 
Mesiolin- 
gual Cusp 






LOWER SECOND MOLAR. 

Calcification Begins, about the Fifth Year. 

Calcification' Completed, Sixteenth to Seventeenth Year. 
Erupts, Twelfth to Sixteenth Year. 
Average Length of Crown, .27. 

Average Length of Root, .50. 

Average Length over All, .78 

This molar differs in so many particulars from the lower. first molar 
that a separate description will be called for. The principal variation 
is usually found in the absence of the fifth lobe or cusp,* resulting in the 
production of an occlusal surface much less complicated. 

The Occlusal Surface of the Crown (Fig. 136). — When the crown 
is studied by looking directly upon this surface, the variations between 

this and the first molar are 
readily noted. Four equally 
proportioned cusps are observed, 
separated from each other by 
Distobuccai four developmental grooves. A 
single pit or fossa is present, the 
four grooves arising from this 
one point. In general outline 
two principal varieties exist: one 
in which the opposite sides of 
the crown are nearly of the same 
length, and parallel with each 
other, with the angles rounded; 
the other, in which either the 
buccal or lingual margin is the longest, with the mesial and distal 
margins converging one way or the other, as the case may be. The 
marginal ridges are formed in a manner similar to those of the first molar, 
with the exception of the distal portion of the buccal ridge, which is not 

*When five cusps are present, the anatomy of this surface does not differ from that of the 
first molar. 




Bucco- 
marginal 
Ridge 
Mesiobuc- 
cal Cusp 



SO ~,~ 



-Lower Second Molar, Occlusal 
Surface. 



THE LOWER MOLARS. 



193 



Distobuccal Buccal Mesiobuccal 
Cusp Groove Cusp 



Distal 
Surface 




Mesial 
Surface 



Fig 137. — Lower Second Molar, 
Buccal Surface. 



broken by a developmental groove. Each marginal ridge is divided 
near its center by one of the grooves of development, the mesial groove 
crossing the mesiomarginal ridge, the buccal groove crossing the bucco- 
marginal ridge, the lingual groove 
crossing the lingual ridge, and the 
distal groove passing over the distal 
ridge. In many instances numerous 
supplemental grooves are present, 
which in turn form a number of 
smaller ridges. The four cusps are 
the mesiobuccal, distobuccal, mesio- 
lingual, and distolingual. In a gen- 
eral way they are similar to the cusps 
of the first lower molar, excepting 
that they are somewhat larger and 
probably less pointed and less angu- 
lar. Each cusp is provided with a 
number of ridges, which descend from 
the summit to the base, two of these contributing to the formation of 
the marginal ridges, one passing to the buccal or lingual, and one, the 

triangular ridge, descending the cen- 
tral incline of each cusp. The names 
given to these various ridges are iden- 
tical with those of the first molar. 

The Buccal Surface of the 
Crown (Fig. 137).— The principal 
difference between this and the cor- 
responding surface of the first molar 
is that produced by the absence of the 
fifth cusp, the surface being divided 
into two parts instead of three. The 
single division is caused by the buccal 
groove, which reaches the surface after 
crossing the buccomarginal ridge 
from the occlusal surface. The posi- 
tion of this groove is usually a little 
to the mesial of the center of the 
surface. Like the buccal groove of the first molar, it may disappear 
gradually as it passes toward the cervical line, or it may end in a well- 
marked pit — the buccal pit. 

I 3 



Lingual 
Groove 




Fig 138. — Lower Second 
Molar, Lingual Surface 



194 



ANATOMY. 



The Lingual Surface of the Crown (Fig. 138).— This surface 
so closely resembles the corresponding surface of the first molar that 
it is somewhat difficult to distinguish one from the other. The occlusal 



V 

~- 2 3 
.2(3 .2 .2<3 



Mesiobuc- 
cal Angle 




Lingual 
Grove 



Fig, [39. Lower Second Molar, Mesial Surface. 

margin may be a trifle less irregular, and in some instances more ex- 
tensive, than the buccal surface, this latter feature seldom occurring in 
the first molar. 

Mesial Groi 








Fig. 140. — Lower Second Molar, Distal Surfac e. 

The Mesial Surface of the Crown (Fig. 139). — This surface cor- 
responds to the mesial surface of the first molar, being flattened or 
slightly convex from buccal to lingual, with an inclination to a slight 
depression or concavity near the cervical margin. 



THE LOWER MOLARS. 



195 



The Distal Surface of the Crown (Fig. 140). — On account of 
the absence of the fifth cusp, this surface is less complex than that of 
the first molar. It is convex in all directions; in most instances smooth, 
in others broken by the distal groove, which reaches it after crossing 
the distomarginal ridge from the occlusal surface. 

The Roots of the Tooth. — Like the first molar, these are two in 
number, a mesial and a distal. They are much less constant in form, 
are often nearer together, and in some instances united. When the 
two roots exist — which may be considered the normal condition — they 
are less flattened upon their mesial and distal sides, with the longitudinal 
depression wanting or but slightly apparent. These roots, therefore, 
are more rounded in general, taper more gradually from neck to apex, 
and end in a rounded apex, this often being provided with a slight distal 
curve. 

LOWER THIRD MOLAR. 

Calcification Begins, Eighth to Ninth Year. 

Calcification Completed, Eighteenth Year. 
Erupts, Sixteenth to Twentieth Ve\r. 
Average Length of Crown. .26. 

Average Length of Roots, .36. 

Average Length over All, .62. 



This tooth is probably subject to a greater variety in form than 
any other. There are, however, two varieties which are most frequently 
met with. In one the crown of the 
tooth is similar to the lower second 
molar, being provided with four 
cusps, which are separated from one 
another by four developmental 
grooves (Fig. 141). The other is 
similar to the lower first molar, 
having five cusps and five develop- 



mental grooves. 




Mesio- 
mar- 
ginal 

Ridge 



While these two forms are those 
most commonly met with, the oc- 
clusal surface may be so broken by 
numerous supplemental and devel- 

that even six or Fig. 

well-defined cusps may be 
present. Whatever complications may exist upon the occlusal surface, 
a central fossa is usually present, from which radiate the various develop- 



opmental grooves 
eight 



rao 



141. — Lower Third Molar, 
Occlusal Surface. 



196 



ANATOMY. 



mental grooves. When the central fossa is absent, the space which it 
should occupy is usually taken up by a rounded cusp, by the interference 
of which the grooves are prevented from uniting, and their course is 
much distorted. Along with these variations, the tooth is subject to 
much variety in size. In some instances the crown is one-third less 
in circumference than that of either the first or second lower molars, 
while in others it is a trifle greater. The increase in the size of the crown 
is generally accompanied by an increase in the number of cusps. One 
feature very common to the crown is its inclination to the circular form, 



Disto- 
lingual 
Cusp 



Disto- Mesio- 

buccal buccal 

Cusp Cusp 



Buccal 
Groove 





Fig. 142. — Lower Third Molar, 
Buccal Surface. 



FlG. M.v~ Lower Third Molar, 
Lingual Surface. 



almost resulting in the absence of the angles common to molars in general. 
The marginal ridges are, of course, subject to the ever-varying con- 
ditions to be found upon the occlusal surface; in general, they are poorly 
defined, and are frequently crossed by numerous small supplemental 
grooves, dividing them into many minute tubercles. The latter are 
smooth and strongly convex, with their general outlines much influenced 
by the number of cusps. 

The Roots of the Tooth.— While this tooth is strongly inclined 
to be two-rooted, like the other lower molars, this condition is by no 
means the common one. Like the crown, the roots are probably more 
variable than those of any other tooth. A single conic root may be 



THE LOWER MOLARS. 



I97 



present, or a mesial and a distal root may exist; again, the mesial root 
may bifurcate, thus resulting in three. In some instances, four, or even 
five, branches may be given off from a common base. When more 




Fig. 144. — Types of Lower Third Molars. 

than two roots are present, they are usually much twisted or crooked, 
and, while generally inclined to the distal, are liable to branch in various 
directions. 

A better idea in the variations in this tooth may be had from the 
accompanying illustration (Fig. 144). 



CHAPTER X. 
The Pulp-cavities of the Teeth. 

In the preceding chapters the study of the teeth has been confined 
to their external forms; it will now be necessary to learn something of 
their internal anatomy, and for this purpose various dissections of each 
individual tooth must be made. 

Dissections. — First, a longitudinal dissection of each tooth should 
be made by sawing or filing from labial to lingual in the anterior teeth, 
of from buccal to lingual in the posterior teeth. Second, a longitudinal 
dissection by sawing from mesial to distal. Third, numerous transverse 
dissections by sawing through the crown or root at various points. 

These dissections will expose to view a central cavity with outlines 
closely corresponding to those of the tooth itself. This is called the 
pulp-cavity, and in the vital tooth contains the formative and life-sustain- 
ing substance of th dentin, the dental-pulp. The pulp-cavity is divided 
into two principal parts, that portion within the crown of the tooth being 
the pulp-clnniihi r, while that traversing the root is the pulp-canal. At 
the apex of the root the canal ends in a small foramen, the apical fora- 
men,* which transmits the blood-vessels and nerves to the pulp. The 
pulp-chamber occupies the center of the crown and is always a single 
cavity; the pulp-canals are prolongations from this central cavity, and 
are usually one for each root, although in some instances two or more 
canals are present in a single root. The form of the pulp-chamber 
varies with the shape of the crown, the outline of the cutting-edge in the 
incisor teeth being reproduced in that part of the chamber nearest to the 
cutting-edge, while in the bicuspids and molars the occlusal surface is 
reproduced on the wall of the pulp-chamber, immediately beneath it. 
the lateral walls corresponding to the various sides of the tooth. In 
the incisors and cuspids the pulp-chamber passes so gradually into the 
pulp-canal that a positive line of demarcation between the two is not 
observed. In the bicuspids and molars the canals may be readily 
distinguished by a sudden constriction and branching out of the cavity 
into the vairous roots, which prolongations gradually decrease in size 

*In many instances there is more than one foramen. 

198 



THE PULP-CAVITIES OF THE TEETH. IQQ 

until the apical foramen is reached. The size of the pulp-cavity is much 
influenced by the age of the tooth, its functional activity, character of 
the occlusion, etc. The tooth-pulp, as the formative organ of the dentin, 
gradually decreases in size as the tooth develops (see Development of 
the Teeth), and as a result of this action the youngest teeth are provided 
with the largest pulp-cavities. At the time of eruption of a tooth, the 
diameter of the pulp-cavity is about equal to one-half the diameter of 
the crown, while the length of the canal must, of necessity, accord with 
the extent of root-calcification. As the growth of the tooth proceeds, 
the diameter of both the chamber and canal is gradually diminished; 
this gradual reduction in size is continued during the life of the tooth, 
and if permitted to proceed until old age, the chamber and canal may 
become almost or entirely obliterated. It must be remembered that 
while the diameter of the root-canal is diminished with the growth of 
the tooth, its length increases, continuing to do so until the time of 
complete root-calcification. During the period of root-development the 
diameter of the root-canal is greatest at the free or apical end of the root, 
at which point it presents a funnel-shaped opening (Fig. 141). As the 
root continues to calcify, this funnel-shaped extremity of the canal 
advances in the direction of calcification, and finally, as the formative 
process nears completion, the mouth of the funnel gradually disappears, 
and the apical foramen is established. The various lobes of the teeth 
are penetrated by a prolongation of the pulp-cavity, these being called 
the horns of the pulp- chamber. The depth to which the horn penetrates 
the lobe varies in accordance with the form of the latter. If the tooth 
is one provided with long, penetrating cusps, the horns of the pulp- 
chamber will also be long, but if the cusps be poorly formed, the horns 
of the chamber will be short. In the anterior teeth, when the lobal 
construction is outlined by well-marked developmental grooves, the 
horns of the pulp-chamber will be three in number and directed toward 
the cutting-edge. These are most marked in young teeth, and gradually 
disappear as age advances. The functional activity of the teeth also 
serves to materially reduce the size of the pulp-chamber. Thus, when 
opposing teeth occlude squarely and firmly against each other, with more 
or less -rubbing or sliding during mastication, the external surface is 
prone to rapid abrasion, and, as a direct result of this external change, 
the pulp-chamber undergoes a corresponding alteration by^a growth 
of secondary dentin about its walls. 



200 ANATOMY. 

THE PULP-CAVITIES OF THE UPPER TEETH. 

Upper Central Incisor. — Figure 145 represents a number of 
labiolingual sections presenting the relative size and shape of the pulp- 
cavity in the upper central incisor at various ages. In No. 1 the con- 
dition existing at about the sixth year, or at a time immediately prior 
to the eruption of the tooth, is shown. The tooth-crown is fully formed 
and calcified; the cervical line may be observed, as well as a small portion 
of the root-wall. The pulp-chamber, which is represented by the dark 




Fig. 145. — The Pulp-cavity in the Upper Central Incisor, from the Sixth to 

the Tenth Year. 

portion of the cut, occupies about one-third of the diameter of the crown 
at its greatest width. The pulp-chamber at this age, when viewed 
in this direction, forms almost a perfect cone, the base of which is directed 
upward or toward the future extremity of the root, and its apex down- 
ward in the direction of the cutting-edge of the tooth. The apex of the 
cone may end somewhat abruptly, or it may be lengthened into a slender, 
horn-like projection, extending well toward the cutting-edge. No. 2 
represents the same tooth about the seventh year, or at a time shortly 
after its eruption. The pulp-chamber has become slightly reduced in 
its basal diameter, while but little change has taken place in the apex. 
That portion of the pulp-cavity above the cervical line represents a part 
of the future pulp-canal. At this age the canal is a direct continuation 
of the conic pulp-chamber, ending above in a broad, funnel-shaped 
extremity. No. 3 shows the condition of the cavity about the eighth year. 
The diameter of the pulp-chamber is considerably diminished, the apex 
has slightly receded, and the horn-like projection has partly disappeared. 
The increase in the length of the canal is about 3/16 of an inch over its 
length at seven years. The two parallel sides of the canal have lengthened 



THE PULP-CAVITIES OF THE UPPER TEETH. 201 

proportionately, and the funnel-shaped extremity is reduced in diameter 
owing to the gradual narrowing of the roots-wall. No. 4 gives the 
relative size of the pulp-chamber and canal at the ninth year, or at a 
time when root-calcification is nearing completion. The decrease in 
the capacity of the chamber is readily apparent; the horn-like projection 
has disappeared and the parallel sides of the canal are partly extended 
into the chamber, thus reducing the length of the cone. In the canal a 
greater reduction has taken place in its diameter, while its length has 
increased about 1/4 of an inch over that at eight years, and the diameter 
of the funnel-shaped opening is but little greater than that of the body 




Fig. 146. — -Pulp-cavity in the Upper Central Incisor. 

of the canal. No. 5, which represents a section of the tooth about the tenth 
year, shows calcification in the root completed, and the apical foramen 
established. A glance at the illustration will show the gradual decrease 
in the capacity of the pulp-cavity and the completion of its growth in an 
apical direction. At this stage of development the fan-shaped extremity 
of the canal gradually disappears, and for the first time in the life of the 
tooth the canal partakes of the external root form throughout its entire 
extent. 

Figure 146, A, represents the size and form of the average pulp- 
cavity in the adult upper central incisor. In its entirety it represents a 
double cone, with a common base near the cervical line, the pulp-cavity 
forming one cone and the pulp-canal the other. At this common base 
the cavity assumes its largest diameter, which measurement is approxi- 
mately equal to one-fourth the labiolingual diameter of the tooth. The 
extent and form of the lower cone, or that represented by the pulp- 
chamber, varies in the adult tooth with the tooth type. Thus, in the 
nervous type the cone is long and narrow, with the apex ending in a hair- 



202 ANATOMY. 

like projection. In the tooth of the lymphatic temperament the cone 
is prone to be wide, with its apex ending abruptly. In the sanguine and 
bilious types the form and extent of the cone do not partake of either 
of the foregoing extremes, but, in keeping with the outline of the crowns. 
are intermediate between them. Figure 146, B, represents the average 
condition of the pulp-cavity in the central incisor in advanced age, and 
shows a general reduction in the size of both chamber and canal. A 
further study of the pulp-chamber and canal may be made by a mesio- 
distal section made through the long axis of the tooth (Fig. 146, C). 
The outline of the cavity, viewed in this way, closely follows the outline 
of the crown and root of the tooth. There is no distinct division between 
the chamber and the canal, the former gradually blending into the latter. 
The outline of the entire cavity is that of a single cone, with its base 
directed toward the cutting-edge and its apex in the direction of the 
apical extremity of the root. The lower margin of the pulp-chamber, 
or that nearest the cutting-edge of the crown, is broad from mesial to 
distal and thin from labial to lingual. This margin in the average adult 
tooth is about on a line with the center of the labial surface of the crown, 
and the lateral walls of the cavity as they pass upward converge slightly. 
and finally blend into the walls of the canal at a point somewhat beyond 




Fig 147. rransverse Sections, Root of Upper Central Incisors, Slightly 

Enlarged 

the cervical line. During the early life of the tooth the margin of the 
chamber nearest the cutting-edge presents three well-defined horns, 
corresponding to the three rudimentary lobes found upon the cutting- 
edge at this period. These horns rapidly disappear, and are seldom 
found after the fifteenth year. In certain tooth types, however, the 
mesial and distal horns may continue ] 'resent until adult age, and even 
into middle life, but when this occurs it is not the result of the temporary 
tooth form, but is occasioned by the permanent angular outline of the 
crown. 

Figure 147 represents a number of transverse sections of an upper 
central incisor, showing the outline and relative size of the pulp-cavity 



THE PULP-CAVITIES OF THE UPPER TEETH. 203 

in passing from the base of the crown toward the apex of the root. No. i 
shows the outline of the cavity at the cervical line; No. 2 represents the 
condition 1/8 of an inch nearer the apex of the root; No. 3 is from the 
center of the root length, while No. 4 is from the region of the apex. 

Upper Lateral Incisor. — The pulp-cavity in the upper lateral 
incisor is so nearly identical with that of the central that it will only be 
necessary to call attention to one or two points which are at variance. 
Figures 148 shows the five stages as represented by the growth of the 
tooth. In general it will be observed that the cavity is much smaller 
than that of the central incisor, but this difference is to be accounted for 




Fig. 148. — Pulp-cavity in the Upper Lateral Incisor, from the Sixth to the 

Tenth Year. 

in the smaller proportions of the tooth. No. 1 shows the condition of 
the crown and pulp-cavity about the sixth year, the pulp-cavity occupying 
a large portion of the partly clacified tooth-crown. No. 2 represents 
the conditions present at the seventh year, or about the time of the 
eruption of the tooth. The pulp-chamber at this age resembles a perfect 
cone, the base of which reaches to the root-walls, and faintly outlines 
the beginning of the future pulp-canal. In No. 3, at eight years, the 
length of the root has increased about 3/16 of an inch, and the parallel 
sides of the walls of the pulp-canal have made their appearance. In No. 
4, at nine years, by the growth of the root the canal has considerably 
increased in length and at the same time much decreased in diameter, 
while in No. 5, at ten years, the root is completely formed, the apical 
foramen established, and the maximum size of the entire pulp-cavity 
in the fully formed tooth shown. 

Figure 149, A, shows the average condition of the pulp-cavity in 
the upper lateral incisor at adult age, while figure 149, B, represents 
the same tooth in old age. In a mesiodistal section — figure 149, C — a 
very close resemblance to the pulp-cavity in the central incisor will be 



204 ANATOMY. 

noticed. While the pulp-cavity is smaller than that of the central 
incisor, it is usually a trifle larger in proportion to the size of the tooth. 
Owing to the marked constriction at the neck of this tooth, there is 
occasionally found a slight line of distinction between the pulp-chamber 
and canal, but in the majority of instances this is not to be observed. 
The horns of the pulp-chamber are in every respect similar to those 
of the central incisor, excepting when they exist permanently, in which 
case the mesial horn is usually the longest. By the transverse sections 
shown in figure 149, the gradual decrease in size and change in form in 
the root-canal are presented, the sections being similar to those made 
in the root of the central incisor. 




Fig. 149. 

Upper Cuspid. — The pulp-cavity of this tooth is in general similar 
to that of the incisors, excepting that the coronal extremity of the chamber 
is conic and inclined to a horn-like projection which penetrates the single 
cusp of the tooth-crown in the direction of its summit. Figure 150 
represents a number of labiopalatal sections. No. 1 shows the condi- 
tion of the pulp-cavity about the seventh year, or fully five years before 
the eruption of the tooth. The pulp-chamber partakes of the cone shape 
previously referred to, but the margins, instead of being straight lines, 
are somewhat bowed or concave, thus conforming more closely to the 
outline of the crown. The central horn of the chamber is proportionately 



THE PULP-CAVITIES OF THE UPPER TEETH. 205 

longer than that of the incisors, in correspondence with the cusp of the 
tooth. At this age the formative process has barely extended to the 
root-walls; therefore, the width of the cavity it about equal to its length. 
In No. 2, at eight years, an increase in the capacity of the chamber over 
that of the incisors is shown, this being the result of the greater bulk in 
the tooth-crown. The cone-like outline of the chamber is somewhat 
broken by an effort of its margins to follow the outline of the crown. 
In No. 3, at nine years, the principal change has taken place in the 
canal, which has lengthened fully 3/16 of an inch, and the funnel-shaped 
extremity, instead of joining with the pulp-chamber direct, is continued 
below by two parallel walls to the true beginning of this cavity. x\t ten 



red 


M 



Fig. 150. — Pulp-cavity in the Upper Cuspid, from the Seventh to the Twelfth 

Year. 

years, No. 4, a more marked transformation has taken place in both 
portions of the cavity. The diameter of the chamber at the cervical line 
has diminished, as has also the length of the cone. The increase in the 
length of the root, which has been proportionately greater than that of 
the preceding year, has extended the length of the canal about 3/8 of 
an inch. The walls of the canal are no longer parallel with each other, 
but are inclined to follow the root-outlines. The funnel-shaped opening 
is much reduced both in length and breadth. No. 5 represents the 
condition at the time of the eruption of the tooth, or about the twelfth 
year. The general outline of the pulp-cavity is that of a double cone, 
with a common base at a point nearly corresponding to the cervical line. 
The diameter in both the chamber and canal has considerably decreased, 
while the central horn in the former has further receded. The calcifica- 
tion of the root externally is about complete and the foramen formed. 
In this particular the cuspid tooth differs from the incisors, and in fact 
from all other teeth, in having its root-calcification about completed and 



2o6 



ANATOMY. 



the apical foramen established at or soon after the time of its eruption. 
Figure 151, A, gives an idea of the capacity of the pulp-cavity in the upper 
cuspid at maturity, while figure 150, B, shows the condition in advanced 
age. Figure 151, C, is a mediodistal section of a matured upper cuspid. 
The coronal extremity of the pulp-chamber is scpaare, and but little 
inclined to follow the outline of the mesial and distal cutting-eds^. 
The chamber passes into the canal without a mark of separation, and 
the latter gradually diminishes in diameter as the apex of the root is 




D 



Fig. ici. 



approached. At its point of beginning the canal is sometimes inclined 
to flatness from mesial to distal, but in passing toward the apex this 
tendency disappears, and it becomes more circular in outline. In figure 
151, D, a transverse section through the tooth at the cervical line is 
shown, giving an idea of the proportionate size and form of the canal in 
the adult tooth, while E, F, and G represent transverse sections through 
the root of the same tooth at various points between the cervical line 
and the apex of the root. 

Upper First Bicuspid. — The study of the pulp-cavity in this 
tooth differs in many particulars from that of the incisors and cuspids. 
First, the line of distinction between pulp-chamber and the root-canal 



THE PULP-CAVITIES OF THE UPPER TEETH. 207 

or canals is, in most instances, definitely marked by the bifurcation of 
the roots and a corresponding branching of the pulp-cavity into two 
line canals, one of which occupies the center of each root. This division 
of the cavity brings the center of the pulp-chamber almost on a level 
with the cervical line. In figure 152, No. 1 shows the partly calcified 
crown of the upper first bicuspid at the seventh year. A portion of the 
pulp-chamber alone may be studied at this period, and this is found to 
be somewhat irregular in outline, with a broadened, funnel-shaped open- 
ing above, and two small, cone-like projections below, pointing into 
either cusp of the crown. These latter projections are the horns of the 
pulp-chamber, and are named in accordance with the cusp which they 




Fig 152. — Pulp-cavities of the Upper First Bicuspid, from the Seventh to the 

Twelfth Vear 

occupy. In very young teeth it is not unusual to find these horns pene- 
trating the dentin almost to the enamel-wall. No. 2, at eight years, 
shows the crown fully calcified and the outline of the base of the roots 
established. The horns of the pulp-chamber have slightly receded, and 
the branching of the canals is made manifest by the central deposit of 
dentin. In No. 3, at nine years, the capacity of the pulp-chamber is 
much decreased, and appears to have receded bodily rootward. The 
roots are calcified to about one-third their full length, and the canals 
which traverse them are each provided with the funnel-shaped opening 
at their free calcifying extremities. In No. 4, at ten years, the decrease 
in the size of the pulp-chamber is not only caused by the deposit of 
dentin upon the occlusal and lateral walls, but from the direction of the 
roots as well. The diameter of the root canals is much less than at nine 
years, but the walls are as yet parallel. No. 5 shows the roots fully 
formed and the apical foramina established, which condition occurs 
about the twelfth year. The horns of the pulp-chamber have receded 



208 



ANATOMY. 



somewhat, and the center of this cavity is now almost on a level with 
the cervical line. The canals have assumed the form of the roots them- 
selves, and their diameter is much diminished. The illustration shows 
the proportionate maximum size of the chamber and canals in this tooth 
after completion of surface calcification. It will be observed that the 
foramina are proportionately smaller than those of the incisors and 
cuspids at a corresponding period, this condition resulting from the 
lesser diameter of the roots. 

Figure 153, A, illustrates the approximate size and form of the 
pulp-chamber and canals at adult age, and attention is called to the 

A B C D 




F. F G 

Fig. 153. — Pulp-cavities of the Upper First Bicuspid, Enlarged about One-third. 



appearance of the horns of the pulp-chamber. It will be observed that 
the horn which penetrates the buccal cusp is larger and more pointed 
than that directed toward the lingual cusp; this condition is fully ex- 
plained by the buccal cusp being proportionately larger and longer than 
the lingual. In the same figure, B represents the pulp-cavity in the first 
upper bicuspid at advanced age. The foregoing description applies 
only to the two-rooted bicuspids, but as many of these teeth have but 
one root, an additional description will be necessary. When a single 
root is present, many varieties in the outline of the pulp-cavity will 
be presented; this variation, however, seldom affects the capacity or 



THE PULP-CAVITIES OF THE UPPER TEETH. 200. 

form of the pulp-chamber. Two distinct canals may exist in the single 
root (Fig. 153, C), branching off from the chamber,ione from the buccal and 
one from the lingual portion. These canals gradually taper in the direction 
of the apex of the root, and may end in a single foramen, or in distinct fora- 
mina. Occasionally the canals will unite before reaching the root-apex and 
continue as a single canal ending in a single foramen, or they may com- 
municate at one point and again diverge and finally end in separate 
foramina. In some instances the pulp-canal appears to be a direct 
continuation of the pulp-chamber, extending throughout the length of 
the root in the form of a flattened canal, with its greatest diameter from 
buccal to lingual (Fig. 153, D). When two separate canals exist in the 
single root, the outward appearance of the root indicates a near approach 
to two roots; when the single flattened canal is present, the root is also 
flattened and shows no sign of bifurcation. Reference has been made 
to the horns of the pulp-chamber, and in this connection it will be well 
to speak of the extent to which they may exist. In that type of tooth 
provided with long penetrating cusps the horns will dip well down into 
the cusp occasionally to the full depth of the dentin, and in rare instances 
may penetrate the enamel. In those teeth lacking in cusp-formation 
the length of the horns will be correspondingly reduced, and may be 
entirely wanting. In the two-rooted upper first bicuspid the floor of 
the pulp-chamber, or that part of the cavity directed rootward, is promi- 
nent and rounded in the center, from which point it gradually slopes 
toward the entrances to the canals, one of which arises from the extreme 
buccal margin, and the other from the extreme lingual margin. Figure 
153, E, represents a transverse section of the two roots immediately 
below the point of bifurcation. F represents a section of the two roots 
midway between the cervical line and the apical extremity, while G is 
a transverse section of D at the cervical line. 

Upper Second Bicuspid. — The pulp-cavity of this tooth is in 
many respects similar to that of the first bicuspid, the principal variations 
being in the horns of the chamber, which are proportionately smaller 
in correspondence with the diminution in cusp-formation. There is 
usually no positive line of demarcation between the chamber and canal, 
the latter being quite large, and broad from buccal to lingual. The 
extent of the pulp-chamber is sometimes well defined by the presence of 
two root-canals, similar to those described in connection with the first 
bicuspid. In rare instances the tooth may possess two roots, each of 
which would be traversed by a canal. Figure 154 represents the various 
stages of the development of the pulp-cavity, as shown by a longitudinal 
14 



2IO 



ANATOMY. 



section from buccal to lingual. No. i shows the condition at seven 
years, or at a time when a portion of the crown only is calcified, in con- 
sequence of which the pulp-chamber alone can be studied at this period. 




Fig. 154. — Pulp-cavities in the Upper Second Bit uspid, from the Seventh to 

the Twelfth Year. 

The buccal and lingual horns of the chamber may be observed penetrat- 
ing the dentin in the direction of their respective cusps. No. 2 shows 
the advance made in the formative process by the eighth year, or at a 




Fig. 



155- 



A 15 

-Section of Upper Second Bicuspid, Slightly Knlarged. 



time immediately prior to the eruption of the tooth; the outline of the 
chamber is completed and the walls of the future pulp-canal faintly 
outlined. At this period there has been but little change in the horns 



THE PULP-CAVITIES OF THE UPPER TEETH. 211 

of the pulp-cavity. No. 3 shows the condition of the tooth at the ninth 
year, or at the beginning of its eruptive period. The diameter of the 
chamber has somewhat decreased, the horns have slightly receded, the 
funnel-shaped extremity of the cavity has advanced beyond the cervical 
line, and is now confined to the canal alone. No. 4 represents the 
condition of the pulp-cavity about the tenth year. A gradual decrease 
in the diameter of both the chamber and canal is observed, and the horns 
of the pulp-cavity are growing less prominent. The length of the root 
having increased nearly one-quarter of an inch, we find a corresponding 
addition to the length of the canal. No. 5 shows the maximum size of the 
pulp-cavity in the upper second bicuspid, which condition accompanies 
the completion of the external calcification at the twelfth year. As 
previously stated, the cavity, in its entirety, presents no line of separation 
between the chamber and canal, but gradually tapers from its broadened 
base in the crown to its ending at the apex of the root. In this tooth, 
as well as in all those previously described, the apical foramen at the 
time of completion of root-calcification is comparatively large, and 
readily penetrated during operations upon it. Figure 155 illustrates 
a number of sections of an upper second bicuspid at maturity. The 
same figure also represents two transverse sections, A being at the cervical 
line, B midway between the cervical line and the apex of the root. 

PULP-CAVITIES OF THE UPPER MOLARS. 

The inner anatomy of the molar teeth being much more complicated 
than any of those previously described, it will be found necessary to make 
a number of dissections in various directions in order to obtain a com- 
prehensive idea of the location and form of the different parts of the 
pulp-cavity. The line of demarcation between the pulp-chamber and 
canals is always definite, the former occupying a central position in the 
crown and seldom extending beyond the cervical line, while the latter 
are given off from the floor of the chamber and penetrate the various 
roots, their entrances being marked by small funnel-shaped openings in 
the floor of the chamber. In the matured tooth the form of the chamber 
usually corresponds to that of the crown of the tooth. The lateral walls 
of the chamber are four in number, and are named according to their 
location — mesial, distal, buccal, and lingual. The average thickness 
of these walls at maturity is about equal to the diameter of the pulp- 
chamber. In that type of tooth common to the lymphatic temperament 
where there is but little constriction at the neck, resulting in the various 



212 



ANATOMY. 



sides of the tooth-crown being nearly parallel with each other, the pulp- 
chamber is nearly quadrilateral in form; but in those teeth marked by a 
decided constriction at the neck, most marked in the nervous tempera- 
ment, the extent of surface covered by the floor of the chamber is much 
less than that occupied by the occlusal portion. In the former class, 
the entrances to the various canals are much farther apart than in the 
latter. The occluding wall is usually much thicker than the lateral 
walls, and is penetrated by the horns of the pulp-chamber, one of which 
extends into each cusp. As in the bicuspids, the extent to which the 
horns penetrate the cusps is controlled by the prominence of the latter. 

The floor of the pulp-chamber is irregu- 
c larly rounded, being high in the corner 
and gradually falling away in the direc- 
tion of the canals. The entrances to the 
B 1 root-canals, three in number, are placed 
in the form of an irregular triangle, called 
t|L ' M the molar triangle. The mesial side of 

A "\ the triangle is usually the longest, the distal 

|^te^ next in length, and the buccal the shortest. 

In young teeth the entrances to the canals 
are usually in the form of funnel-shaped 
openings, are comparatively easy of access, 
but after maturity may disappear and be but little larger than the canals 
themselves. To properly study the position occupied by the entrance 
to the canals on the floor of the pulp-chamber, a transverse section of 
the tooth should be made at a point somewhat above the cervical line, 
at the same time preserving both the crown and the roots of the tooth 
for comparison. The entrance to the lingual canal, which is usually 
the largest and most readily accessible, may be located by a line drawn 
through the center of the occlusal surface of the crown (Fig. 156) from 
buccal to lingual, A, and by another line drawn from mesial to distal 
almost parallel with the linguomarginal ridge, passing through the 
summits of the mesiolingual and distolingual cusps, B; the point at which 
these two lines intersect will mark the approximate location of the lingual 
canal. The entrance to the mesiobuccal canal may be located by a line 
drawn from the inner side of the mesiobuccal angle to a corresponding 
position near the distobuccal angle, C. This should be intersected 
by a line drawn from the summit of the mesiobuccal cusp to the summit 
of the mesiolingual cusp, D, the point at which these two lines cross 
marking the entrance to the mesiobuccal canal. The location of the 




THE PULP-CAVITIES OF THE UPPER TEETH. 213 

entrance to the distobuccal canal is found by the line, C, which is inter- 
sected by another line, E, drawn from the summit of the distobuccal 
cusp to a corresponding point on the distolingual cusp. The nearer 
the tooth-crown approaches to the quadrilateral, the nearer will the 
molar triangle approach the equilateral. 

Upper First Molar. — In the dissection of this tooth, the pulp- 
chamber and two of the root-canals only can be shown, but these will 
be sufficient to pursue the study with intelligence. Figure 157 shows 
a number of longitudinal sections, made in such a manner as to expose 
the lingual canal, usually the largest, and the mesiobuccal canal. No. 1 
illustrates the approximate size and form of the pulp-chamber at the 




Fig. 157. — Pulp-cavity of Upper First Molar, from the Fifth to the Ninth 
Year. Lingual and Mesiobuccal Canals. 

fifth year. At this period the chamber occupies a large proportion of 
the center of the tooth-crown. Two of the four horns are seen, one of 
which penetrates the mesiobuccal cusp, and one the mesiolingual cusp. 
In many instances the horns of the molar teeth are quite slender, penetrat- 
ing the dentin to a greater depth than that shown in the illustration, in 
the form of minute hair-like projections, which in some instances reach 
almost to the enamel walls. 

No. 2 illustrates the condition of the pulp-cavity at the sixth year, 
or at the time of eruption. The outline of the pulp- chamber is completed, 
and the floor has begun to make its appearance by a central deposit 
of dentin. It will be observed that the lateral walls of the chamber are 
somewhat less in thickness than the occluding wall, a condition which 
will become more pronounced as the tooth develops. With the beginning 
of the formative process in the floor of the chamber we find the trifurca- 
tion of the roots established, and the beginning of the canals outlined. 
The canals at this period are quite similar to those of the bicuspid, being 
provided with a funnel-shaped extremity, which extends from the free 



214 • ANATOMY. 

calcifying margins of the roots to the floor of the chamber. No. 3 shows 
the change which has taken place at the seventh year. While the pulp- 
chamber is somewhat reduced in size, but little change is noticeable 
in its outline. By this time the floor of the chamber has become an 
important factor in the tooth development. By the constant lateral 
extension of this central deposit of dentin the floor of the chamber is 
gradually spread out, this alteration being at the expense of the entrances 
to the root-canals, which become reduced in diameter as the floor is 
extended. The horns of the chamber are slightly less prominent, but 
this part of the cavity has the appearance of having receded bodily root- 
ward. The roots have advanced somewhat beyond the point of trifurca- 
tion, and a definite outline has been given to the canals. At this period 
the diameter of the root-wall is about equal to the diameter of the pulp- 
canal. Along with the gradual decrease in the diameter of the roots, 
there is observed a corresponding decrease in the width of the funnel- 
shaped extremities of the canals. 

At the eighth year, No. 4, a gradual reduction in the capacity of 
both the chamber and canal is noted. Accompanying the above con- 
dition there is found a corresponding increase in the thickness of the 
surrounding walls. The horns of the pulp-chamber are much reduced 
in size, and the form of the chamber more closely resembles that of the 
general contour of the tooth-crown. The increase in the length of the 
roots is proportionately greater than that of previous years, in conse- 
quence of which the length of the canals is increased to a greater degree. 
In No. 5 the maximum size of the chamber and canals is apparent, which 
condition takes place about the ninth year, or at a time when calcification 
of the tooth is completed externally. In some instances, owing to the 
additional length of the lingual root, the apical foramen may not be 
established before the tenth year. At this latter period it is safe to assume 
that all three canals have completed their longitudinal extent, and the 
foramina, although proportionately large, have been established, so 
that a more definite description of each canal may be given. The lingual 
canal (Fig. 158, A) is usually the largest, and branches off from the floor 
of the chamber, near the mesiodistal center of the extreme lingual margin, 
the entrance in the average tooth being well defined by a circular, funnel- 
shaped opening. The direction of this canal is usually upward and 
slightly inward, until the apical extremity is approached, at which point 
it is inclined to the buccal. The circular form presented at the beginning 
of the canal is generally continued throughout its entire length, in this 
respect differing from the two buccal canals. The average length of 



THE PULP-CAVITIES OF THE UPPER TEETH. 215 

the lingual canal is about 1/2 of an inch. The mesiobuccal canal (Fig. 
158, B) branches off from the floor of the chamber, at its extreme mesio- 
buccal angle, and the entrance, instead of being funnel-shaped and easy 
of access, is flattened from mesial to distal, and frequently difficult to 
enter. This flattened form continues throughout its course, which for 
the distance of 18 of an inch is in a buccal and mesial direction; beyond 
this point it is usually inclined to the buccal, until the upper thrid of the 
root is reached, where it turns rather abruptly to the distal. This canal 
is generally a trifle shorter than the lingual, averging about 3/8 to 7/16 of 




A 



D E F 

Fig 158. — Pulp-cavities of Upper First Molar, Slightly Enlarged. 

an inch. The distobuccal canal (Fig. 158, C) branches off from the floor 
of the chamber at the extreme distobuccal angle. In those teeth which 
most nearly approach the quadrilateral form, the entrance to this canal 
will be farther from the center of the tooth, the molar triangle in this 
instance being almost an equilateral. It sometimes happens that the 
entrance to this canal is directly in the floor of the pulp-chamber, near to, 
but not against, its buccodistal angle. The entrance is usually abrupt, 
seldom being funnel-shaped, making it by far the most difficult of access. 
It is inclined to be circular in form, and more or less tortuous in its course. 
Immediately above the point of beginning it is inclined toward the buccal 
and distal; near its center it may incline slightly to the mesial; and 



2l6 ANATOMY. 

finally, at its upper third, turns somewhat abruptly in a distobuccal 
direction. This canal is usually the shortest of the three, its average 
length being about 3/8 of an inch. 

Figure 158 also illustrates a number of transverse sections of this 
tooth, D being made at the cervical line, looking toward the crown, E 
looking toward the roots, while F represents a transverse section at a 
point immediately above the floor of the pulp-chamber. 

Upper Second Molar. — In many respects the pulp-chamber of 
this tooth is similar to that of the first molar, but there are a few variations 
which must be briefly described. First, the outline of the tooth-crown 




9th year nth year 1.3th year 15th year 1 8th year 

Fig. 159. — Pulp-cavities in the Upper Second M'>!ar,'from the Ninth to the 

Eighteenth Year. 

being much more flattened from mesial to distal, a corresponding varia- 
tion is noted in the form of the pulp-chamber, increasing the length of 
the mesial side of the molar triangle, and decreasing the length of the 
buccal and distal sides. The chamber is more or less flattened from 
mesial to distal, making it somewhat oblong from buccal to lingual. 
Second, on account of a reduction in the prominence of the cusps, the 
horns of the cavity are usually somewhat less pronounced than those of 
the first molar. Third, the floor of the cavity is less convex, and slopes 
more gradually toward the entrances of the various canals. In a general 
way, the rules given for ascertaining the approximate location of the 
entrances to the canals in the first molar apply to this tooth. The com- 
parative size and form of the pulp-chamber and canals during the develop- 
ment of the tooth are shown in figure 159, extending from the ninth to 
the sixteenth or eighteenth year, at which latter period the crown and 
roots of the tooth are fully calcified externally. 

Upper Third Molar. — In this tooth the conditions are so variable 
that a description of the pulp-cavity taken from a single tooth would be 
insufficient. In the majority of instances the outline of the tooth-crown 



THE PULP-CAVITIES OF THE UPPER TEETH. 217 

approaches the triangular form, and in consequence the pulp-chamber 
is triangular rather than quadrilateral or oblong. The mesial border 
of the chamber is the longest, the distal next in length, and the buccal 
the shortest of the three. The horns are generally less in number and 
much less pronounced than those of either the first or second molars. 
The floor of the chamber may be broken by irregularities similar to 
those previously described, or it may be entirely absent, this latter con- 
dition occurring when the tooth has but a single root accompanied by a 
single canal. The various stages of development having been given in 
connection with the general description of the tooth, no attempt will 

ABC 




Fig. 160. — Longitudinal Sections, Upper Third Molar, Slightly Enlarged. 

be made to describe this by longitudinal sections, the complications in 
root-form making such a proceeding impracticable. Instead of so doing, 
the space will be devoted to a brief description of the variety of pulp- 
canals found in this tooth. Probably the most frequent condition is 
that which resembles the first and second molars — i.e., three canals 
branching off from the chamber in as many different roots, two to the 
buccal and one to the lingual. When the three canals exist, the entrances 
to them will be well beyond the cervical line, where they will be found 
clustered much closer together than those of the first and second molars, 
this difference in their location being so marked that the diagram pre- 
viously given cannot be depended upon in an attempt to locate them. 
The usual course of these canals is first slightly mesial, then distal, and 
finally in a distolingual direction. On account of the pulp-chamber 
extending well beyond the cervical line, the canals are much shorter than 
those of the first or second molars, their average length being less than 
1/2 of an inch. Another form frequently met with is that of the flattened 
single canal, occurring when the tooth has but a single root, which shows 



2l8 ANATOMY. 

no signs of trifurcating (Fig. 160, A). In this instance the pulp-chamber 
gradually passes into the canal, and the chamber is without a floor. Such 
a canal is shaped like the chamber at its point of beginning; but as it 
passes toward the apex it becomes flattened in the direction of the smallest 
diameter of the root. But little difficulty is experienced in entering 
such a canal, and usually it is readily followed to its apex. Another 
condition frequently met with in the single-rooted third molar is that 
of one or more canals branching off from the floor of the chamber, their 
course through the root-substance being without regard to the external 
contour of the root (Fig. 160, B). These canals, which may exist to 
the number of five or six, are usually very minute, and in some instances 
may pass from the floor of the chamber to the apex of the root almost 
in a direct line, and end in distinct foramina, or they may take a tortuous 
course, and when near the apex unite, ending in a single foramen.' When 
the tooth is provided with four, five, or even six small roots, as sometimes 
occurs, each root will be traversed by a minute canal, the entrances to 
these being variously placed about the floor and lateral margins of the 
pulp-chamber (Fig. 160, C). In all operations upon this tooth it must 
be recalled that it is the last to be calcified, and consequently the canals 
and foramina are proportionately larger than in the other teeth; at the 
same time, it possesses one advantage over the others — i.e., (with the 
single exception of the cuspid), being fully calcified at or about the time 
of its eruption. 



PULP-CAVITIES OF THE LOWER TEETH. 2I( 

PULP-CAVITIES OF THE LOWER TEETH. 

A B C 




Fig. 161. — Pulp-cavities of the Lower Incisor. 

The outline of the pulp-cavities of the lower teeth, like those of the 
upper, corresponds to the general tooth contour. The comparative size 
of the cavity at various stages of tooth development will not be repeated 
in this description, the conditions being similar to those in the upper 
teeth (see also Development of the Teeth). 

Lower Incisors. — The pulp-cavities of the lower central and 
lateral incisors are so nearly alike that a single description will answer 
for both. Figure 160, A, represents a labiolingual section of a lower 
incisor, showing the most frequent form of the pulp-cavity. The tooth 
from which the section was prepared was one about middle life, the 
cavity in younger teeth being proportionately larger, while a gradual 
decrease in diameter would be noted with advancing age. There is no 
mark of distinction between the pulp-chamber and canal, so that an 
imaginary separation would have to be made at the cervical line, or 
slightly below that point. Taken in its entirety, the cavity presents the 
form of a double cone, the common base of which is slightly to rootward 
of the cervical line. The chamber penetrates the crown fully half-way 
to the cutting-edge, at which point it ends in a thin, fan-like margin 
(best observed in mesiodistal section), while the canal gradually decreases 
in size until the apical foramen is reached. Although this is the most 
common form of the pulp-cavity in the lower incisors, it is by no means 
the constant condition. The tooth is not infrequently provided with a 
medium-sized pulp-chamber, which extends somewhat below the cervical 



220 ANATOMY. 

line, beyond which point it branches into fine canals, which are continued 
separately until the apical third of the roots is approached, when they 
again unite, and finally end in a single foramen. Figure 161, C, repre- 
sents a mesiodistal section of a young lower incisor, in which the three 
small horns of the pulp-chamber are apparent. At this period the fan- 
shaped extremity of the pulp-chamber occupies about one-half of the 
mesiodistal diameter of the crown, and the horn-like projections extend 
well toward the enamel cap. Figure 161, B, shows the average size and 
form of the pulp-cavity at maturity, by a mesiodistal section through the 
long axis of the tooth. In this it will be observed that the horns of the 
pulp-cavity have disappeared, and that the capacity of the cavity in 
general is much reduced. 




Fig. 162. — Pulp-cavities of the Lower Cuspids. 

Lower Cuspids. — The pulp-chamber and canal in this tooth, while 
usally conforming to the general contuor of the tooth, are frequently found 
to vary greatly, both in outline and in size. The most common form, 
however, is that shown in Fig. 162, A, a labiolingual section of an adult 
tooth. The chamber and canal have no line of demarcation, and unite 
at a common base considerably below the cervical line, the former pene- 
trating the crown of the tooth to a point about midway between the cer- 
vical line and the summit of the cusp, at which point it ends in a sharp, 
hair-like projection. Accompanying this common form there is much 



PULP-CAVITIES OF THE LOWER TEETH. 



221 



variation in size, even in teeth of the same age. Fig. 162, B, shows 
another labiolingual section of an adult lower cuspid, in which the pulp- 
cavity fails to accurately follow the outline of the tooth, and its capacity 
is much less than that shown at A. The root of this tooth is in most 
instances circular, in which case the canal will be similarly formed; but 
occasionally the root will be much flattened from mesial to distal, and 
as a result of this the canal will also be much flattened. The canal of 
this tooth is seldom divided. In Fig. 162, C, a mesiodistal section of a 
lower cuspid is shown, and it will be observed that the fan-shaped ex- 
tremity of the chamber common to the incisors is absent, the cavity end- 
ing rather abruptly, or by a fine line near the center of the crown. The 
same illustration also shows a number of transverse sections, which will 
give an idea of the form of the cavity at various parts of the tooth. 

Lower Bicuspids. — The pulp-cavities of these teeth may be best 
described collectively, thus affording an opportunity for comparison. Unlike 




Fig. 163. — Sections of Lower Bicuspids. 



the upper bicuspids, it is seldom that the canals are definitely separated 
from the chambers. That part of the cavity within the crown, however, 
is usually quite wide from buccal to lingual, and unites with the canal by 
a long, funnel-shaped constriction. The center of the pulp-chamber 
may be considered as being about on a level with the cervical line. In 
the first bicuspid the pulp-cavity is provided with a single horn, which 
extends with more or less prominence in the direction of the buccal cusp. 
That part of the chamber facing the lingual cusp is usually rounded off. 



222 ANATOMY. 

In the second bicuspid the occlusal wall of the pulp-chamber generally 
presents a different form; two well-defined horns are usually present, of 
which the buccal is the longest; or the chamber may be prominently 
rounded at these points. The pulp-chamber of the second bicuspid is 
generally larger than that of the first. The canals of these teeth are 
usually circular throughout, and are readily penetrated until the apical 
third is reached, beyond which point they are extremely small. In some 
instances the canal divides near the center of the root, and is continued 
as two canals, ending in distinct foramina, or, after separating, they may 
again unite, and end in a single foramen. In Fig. 163 the average size 
and form of the canal in these teeth is shown by a number of mesiodistal 
and transverse sections. 




FlG. 164. — Sections of Lower Molars, Enlarged about One-third. 

Lower Molars. — The form of the pulp-chambers of the lower molars 
corresponds to the general outline of the crown, and the form of the root- 
canals is similar to the general contour of the roots. The pulp-chambers 
approach the quadrilateral form; the buccal and lingual sides are some- 
what the longest, the mesial next in length, and the distal, usually slightly 
rounded, is the shortest. The occluding wall is convex rootward, sloping 
in the direction of the various cusps, each of which is penetrated by a horn. 
Like the horns of these pulp-chambers in general, the extent to which these 



PULP-CAVITIES OF THE LOWER TEETH. 223 

penetrate the cusps is influenced by the age, type, and functional activity of 
the organ. The floor of the cavity is convex in the direction of the occlusal 
surface, but this convexity is principally from mesial to distal. From the 
summit of this convexity the floor slopes to the entrances of the canals, the 
opening into which is inclined to be funnel-shaped rather than abrupt. 
The lateral walls of the chamber are much inclined to follow the general 
contour of the crown. The horns of the pulp-chamber are usually more pro- 
nounced in the first than in the second molar, and still less clearly defined 
in the third than in the second. The roots of the first molar being some- 
what further apart than those of the second, the floor of the chamber in 
the former is slightly more extensive than in the latter. To study the 
pulp-cavities of these teeth a longitudinal section should be made through 
the center of the tooth from mesial to distal. Fig. 164, A, shows such a 
dissection through the first molar, and illustrates the average size and 
form of the chamber and canals at adult age. The canals join the 
chamber by a funnel-shaped opening, and but little difficulty will be found 
in effecting an entrance, but to follow them to their apices will be more 
perplexing. The roots of this tooth being much flattened from mesial to 
distal, the canals are also flattened in this direction, but broad from buccal 
to lingual. The entrances of these canals may be found at the extreme 
mesial and distal margins of the pulp-chamber, and usually extend from 
the buccal to the lingual walls of the cavity. It is not uncommon for the 
mesial canal to divide soon after leaving the chamber, and continue as 
two canals, ending in separate foramina (Fig. 164, B). This condition 
is seldom present in the distal canal, which is usually straight from its 
mouth to the apical foramen. The capacity of the pulp-chamber is 
usually a trifle less than that of the first molar, and the entrances to the 
canals are somewhat nearer together. In other respects the cavity is simi- 
lar to that of the first molar. Fig. 164 also shows a number of transverse 
sections through a lower molar, and gives an idea of the size of the canals 
at various parts of the roots. In some instances the roots of the second 
molar coalesce, in which case a single root-canal may be present. In 
the third molar the most common form of the pulp-cavity is one similar 
to that of the first, but both the chamber and canals are smaller. Unlike 
the pulp-cavity of the corresponding upper tooth, this tooth is not sub- 
ject to so much variation, although it is sometimes found with a single 
root traversed by a single canal, which may be accompanied by a rather 
large pulp-chamber. 



CHAPTER XL 

The Deciduous Teeth, Their Arrangement, Occlusion, Etc. ; Their 
Calcification, Eruption, Decalcification, Shedding Process, 
and Average Measurements ; Their Surfaces, Grooves, Fossae, 
Ridges, Sulci, and Pulp-Cavities. 



THE DECIDUOUS TEETH. 



Central Incisor Lateral Incisor 



Cuspid 



First Molar 



Second Molar 



Upper 



Lower 




Central Incisor Lateral Incisor Cuspid First Molar Second Molar 

Fig. 165. — The Deciduous Teeth, Upper and Lower, from the Left Side of the Mouth. 

As implied by the word deciduous, these teeth are temporary in their 
nature, and, after subserving the purposes of early childhood, are thrown 
off by an operation of the economy to give place to the permanent organs. 
The shedding process takes place in the incisors between the seventh 
and eighth years, in the molars from the tenth to the eleventh years, and 
in the cuspids about the twelfth year. This shedding process, however, 

224 



THE DECIDUOUS TEETH. 225 

does not indicate the period at which the degeneracy of the tooth begins, 
for, in a year or two after the roots are completely formed and the apical 
foramen established, decalcification begins at the apical ends and con- 
t inues in the direction of the crown until absorption of the entire root has 
ta ken place and the crown is lost from lack of support. Decalcification 
in the incisors begins between the fourth and fifth years, in the molars 
from the seventh to the eighth years, and in the cuspids about the ninth 
year. 

The deciduous teeth are twenty in number, ten in each jaw, and may 
be classified as follows: Four incisors, two cuspids, and four molars. 
The incisors, central and lateral, occupy the central portion of the arch, 
are placed two upon each side of the median line, and are succeeded by 
the four permanent incisors, which finally occupy the same position. 
The cuspids are located immediately to the distal of the lateral incisors, 
and are displaced by the permanent cuspids. The first and second molars 
come next in the arch, but, unlike the anterior teeth, are followed by per- 
manent successors of another class, the first and second bicuspids, the 
permanent molars erupting posteriorly to these as the jaw increases in 
length. 

In general the deciduous teeth resemble their permanent successors, 
yet there are a number of minor differences which will require a compara- 
tive description. Both the crowns and the roots are much smaller in 
every direction than those of the permanent teeth, but the diameter of 
the crowns is proportionately greater than that of the roots, while the 
roots are proportionately longer. The fact that the roots are smaller in 
proportion than the crowns is productive of a neck much more constricted. 
The roots of the deciduous teeth are the same in number as those of the 
corresponding permanent teeth, the incisors and cuspids being provided 
with one, the upper molars with three, and the lower molars with two. 



is 



226 



ANATOMY. 



THE OCCLUSION OF THE DECIDUOUS TEETH. 



Cuspid 



Incisors 



First Molar 
Second Molar 



Permanent First 
Molar 




Fig. 166. — The Upper Dental Arch about the Seventh Year. 

The arrangement of the deciduous teeth in the jaws is similar to that 
of the permanent organs, the upper teeth describing the segment of a 
larger circle than the lower, in consequence of which the upper teeth 
close over or outside of the lower. The character of the occlusion in the 
deciduous teeth is not subject to so much variation as that found in con- 
nection with the permanent set, this being accounted for by the more 
constant form in the crowns of the former. The relations existing between 
the upper and lower deciduous teeth when in contact is such that each 
tooth, with the exception of the lower central incisor and the upper second 
molar, occludes with two teeth of the opposite jaw, the upper central 
incisor being opposed by the entire cutting-edge of the lower central and 
the mesial third of the lower lateral; the upper lateral coming in contact 
with the remaining two-thirds of the lower lateral and a portion of the 
mesial half of the lower cuspid, this arrangement continuing throughout 
the series. The foregoing description of the occlusion of the deciduous 
teeth is applicable to but a small part of their transitory existence. By 
the time they are fully erupted and have assumed their respective posi- 
tions in the arch, the increase in the size of the bone is sufficient to create 
a slight space between the teeth, which condition is soon followed by a 
greater separation through the protrusion of the anterior teeth, caused by 
the growth and approach of the permanent teeth from behind. 

The calcification of the deciduous teeth is similar to that of the per- 
manent, the process of the incisors and cuspids beginning along the cut- 



THE DECIDUOUS TEETH IN DETAIL. 



227 



ting-edges in three distinct lobes, while in the molars a center of calcifi- 
cation is provided for each cusp (see Development of the Teeth). 

THE DECIDUOUS TEETH IN DETAIL. 

UPPER CENTRAL INCISOR. 






Eighteenth Month 
after Birth 



Sixth Month after 
Birth 



Fortieth Week 



Twentieth Week 




Fourth Year 



Fifth Year 



Seventh Year. 



O 
c 



Fig. 167. 

Calcification Begins, about the Fourth Fetal Month. 

Calcification Completed, Seventeenth to Eighteenth Month after Birth. 
Erupts, Sixth to Eighth Month after Birth. 
Decalcification Begins, about the Fourth Year. 

Shedding Process Takes Place, about the Seventh Year. 
Average Length of Crown, .23. 

Average Length of Root, .39. 

Average Length over All, .62. 

This tooth, as well as all of the deciduous teeth, presents for examina- 
tion numerous surfaces, margins, and angles, these being the same in 
name and location as those of the permanent teeth. 

The Labial Surface of the Crown (Fig. 168). — This surface is smooth 
and generally convex, but with an inclination to flatness near the incisive 
margin. The mesial margin is slightly convex in the direction of the 
length of the tooth, and rounded from labial to lingual. The distal mar- 
gin is decidedly convex from the cutting-edge to the cervical line, in many 
instances forming almost a complete semicircle, which is usually at the 
expense of the distal angle of the crown. The cervical margin is deeply 
concave in the direction of the root, and the incisive margin is straight over 
its central portion and rounded or angular at its extremities. The labial 
grooves are seldom so well defined as those upon the permanent incisors. 

The Lingual Surface of the Crown. — In some instances this 
surface is smooth and concave near the cutting-edge and convex over the 
cervical portion, with the marginal ridges well defined. In other cases it is 
concave from the cutting-edge to the cervical ridge, being provided with a 
longitudinal ridge in the center, a slight depression upon either side, and 



228 



ANATOMY. 



marginal ridges poorly denned. In the former instance the lingual 
fossa is present; in the latter it is absent. The mesial and distal sur- 
faces of the crown are both smooth and convex, the 
former being inclined to flatness over its cervical 
third — a condition which is seldom present in the 
latter. The mesialangle is alone well defined, the 
cutting-edge passing into the distal surface with a 
long, gradual sweep, thus in a measure destroying 
the distal angle. The neck of the tooth is marked 
by a decided constriction, which is principally pro- 
duced at the expense of the crown alone. The 
root of the tooth, when compared with the root of 
the permanent central incisor, is much longer in 
proportion to the length of the crown. In some 
instances it is flattened from mesial to distal, 
these two sides converging as they pass to the lingual; in others it is flat- 
tened from labial to lingual. Generally speaking, it is a single root, 
but is occasionally provided with a slight mesial curve near its apical 
third, and it is sometimes curved slightly from labial to lingual. 




Fig. 168. 



UPPER LATERAL INCISOR. 






Sixteenth Month 
alter Birth 



Sixth Month after 
Birth 



Fortieth Week 
Twentieth Week 




Fifth Year 



Seventh Year 



Eighth Year 



31 
n 
P 



Fig. 169. 

Calcification Begins, about the Fourth Fetal Month. 

Calcification Completed, Fourteenth to Sixteenth Month after Birth. 
Erupts, Seventh to Ninth Month after Birth. 
Decalcification Begins, about the Fifth Year. 

Shedding Process Takes Place about the Eighth Year. 
Average Length of Crown, .25. 

Average Length of Root, .45. 

Average Length over All, .70. 

The various surfaces of this tooth so closely resemble those of the cen- 
tral incisor that a separate description will be unnecessary; in a general 



THE DECIDUOUS TEETH IN DETAIL. 



229 



way, however, there are a few minor points of 
difference. The tooth is smaller in every direc- 
tion excepting in its length, which is generally 
equal to and frequently greater than that of the 
central incisor. The diameter of the root is but 
little less than that of the central, while the 
mesiodistal measurement of the crown is about 
one-third less, in consequence of which the neck 
of the tooth is not so well defined. The angles of 
the crown are more rounded than those of the 
central incisors. 




Fig. 170 — Upper Lat- 
eral Incisor, Labial Sur- 
face. 



UPPER CUSPID. 



« 1 
o 



Second Year 



Sixth Month 
after Birth 



Bir.h 



Thirtieth Week 
Embryo 




Ninth Year 



Tenth Year 



Twelfth Year 



Fig. 171. 

Calcification Begins, about the Fifth Fetal Month. 

Calcification Completed, about Two Years after Birth. 

Erupts, Seventeenth to Eighteenth Month after Birth. 
Decalcification Begins, about the Ninth Year. 

Shedding Process Takes Place, about the Twelfth Year. 
Average Length of Crown, .25. 

Average Length of Root, .53. 

Average Length over All, .78. 



Like the permanent cuspid, the general contour of this tooth is that of 
a double cone, the lines of which are somewhat broken. The greatest 
mesiodistal extent of the crown is from angle to angle, and this measure- 
ment about corresponds with the width of the crown of the central incisor. 

The Labial Surface of the Crown (Fig. 172). — This surface is 
strongly convex from mesial to distal, and slightly so from the cutting-edge 
to the cervical line. It is bounded by five margins: mesial, distal, cervical, 



230 



ANATOMY. 



mesio-incisive, and disto-incisive. The mesial and distal margins are 
rounded and smooth, the cervical well outlined by the cervical line and 
base of the cervical ridge, while the two incisive margins are formed by 
the mesial and distal cutting-edges. The labial grooves are thrown well 
toward the lateral margins, and are usually more distinct than those 
upon the incisors. The labial ridge is prominent. 

The Lingual Surface of the Crown. — This 
surface is generally divided into two portions by 
the lingual ridge, which extends from the sum- 
mit of the cusp to the base of the cervical ridge. 
On either side of this ridge are the lingual 
grooves, but which appear more in the form of 
small fossae. The marginal ridges are fairly well 
defined. 

The Mesial and Distal Surfaces of the 
Crown. — The extent of these two surfaces is fre- 
quently much interfered with by the slope of the 
mesial and distal cutting-edges, which may be so 
long that the angles of the crown are forced 
well toward the cervical line, in some instances almost obliterating 
these two surfaces. When the cutting-edges are shorter, these sur- 
faces present a marked general convexity. While the summit of the cusp 
will always be found to be in a direct line with the long axis of the tooth, 
there is in nearly every instance a difference in the length of the cutting- 
edges, and, unlike the cutting-edges of the permanent cuspid, the mesial 
is usually the longer. The neck of the tooth is much constricted and 
the root straight and conic. 




Upper Cuspid, 



Labial Surface. 



THE DECIDUOUS TEETH IN DETAIL. 



231 



THE UPPER MOLARS. 



Upper First Molar. 



Twenty-two 
Months Old 

Eighteen 
Months 
Old 

One Year 
Old 



Six Months 

after Birth 

, Forty Weeks 




Six Years 
Seven Years 

Nine Years 
Ten Years 



r 

a 
o 

n 

Si 
o 
P 

5' 



Fig. 173. 

Calcification Begins, about the Fifth Fetal Month. 

Calcification Completed, Eighteenth to Twentieth Month after Birth. 
Erupts, Fourteenth to Fifteenth Month after Birth. 
Decalcification Begins, Sixth to Seventh Year. 

Shedding Process Takes Place, about the Tenth Year. 
Average Length of Crown, .20. 

Average Length of Root, .39. 

Average Length over All, .59. 

The contour and lobate construction of the crown ofjthis tooth is 
peculiar to itself, being dissimilar to any other class of teeth in the mouth. 
Calcification takes place from three centers, two for the buccal and one 
for the lingual half of the crown. The general form of the crown may 
best be studied by an examination of the occlusal surface. 

The Occlusal Surface of the Crown. — The outlines represented are 
those of an irregular quadrilateral, of which the buccal and mesial sides 
are the longest. The angles of the quadrilateral are somewhat variable, 
the mesiobuccal being acute, the mesiolingual obtuse, while the two 
distal angles are rounded right angles. The surface is surmounted by 
three cusps, a mesiobuccal, a distobuccal, and a lingual. These various 
cusps are separated from one another by three developmental grooves — the 
mesial, the distal, and the buccal. The marginal ridges are sharp and 
well defined, this being particularly true of the buccal and lingual, which 
resemble cutting-edges. The mesio-marginal ridge begins at the 
mesiobuccal angle, and, after making a long distal curve, ends in the 
mesial incline of the lingual cusp. The center of the surface is deeply 
and irregularly concave, producing the central fossa, and descending 
from the various ridges and cusps surrounding it are numerous supple- 
mental grooves and ridges. The various developmental grooves are not 
inclined to cross the marginal ridges, although in some instances one or 
two may be found to do so. 



232 



ANATOMY. 



The Buccal Surface of the Crown (Fig. 173). — This surface is 
generally smooth and convex, with an excessively developed cervical ridge, 
which is particularly prominent at its mesial extremity. The buccal 
groove is in the form of a slight depression, and the buccal ridges, common 
to all molars, are scarcely to be observed. The mesial, occlusal, and cer- 
vical margins are distinctly outlined, while the distal margin is obliterated 
by the gradual passing of this surface into the distal surface. 

The Lingual Surface of the Crown. — This surface is circular in 
outline, decidedly convex and smooth, and is seldom broken by grooves 

and ridges. It is most prominent 
near the center, from which point 
it slopes in every direction. The 
cervical ridge is not so pronounced 
as that of the buccal surface, but 
there is a sudden rounding of the 
surface in a cervical direction to 
meet the lingual root. 

The Mesial Surface of the 
Crown. — This surface is probably 
more extensive than any of the 
others; it is inclined to flatness, 
with a slight conic convexity over 
its occlusal third, and a slight con- 
cavity near the cervix. The buc- 
colingual measurement of the sur- 
face is nearly twice as great as that from the occlusal margin to the 
cervical line. It is much more prominent near the occlusal margin, so 
that a V-shaped space usually exists between it and the distal surface of 
the cuspid. 

The Distal Surface of the Crown. — The extent of this surface 
is much less than that of the mesial; it presents a general convexity, and 
is seldom broken by grooves or ridges, although occasionally the distal 
groove crosses its occlusal margin. Like the deciduous teeth previously 
described, the neck of the tooth is marked by a decided and abrupt con- 
striction, this form appearing to arise from the heavy enamel folds which 
are present near the cervical line, rather than from any marked con- 
striction in the base of the roots themselves. 

The roots of the tooth are three in number — a mesiobuccal, a disto- 
buccal, and a lingual; of these, the latter is usually the largest and longest. 
The two buccal roots are much flattened from mesial to distal, while the 




Fig. 174. — Occlusal Surfaces of the 

I )c( iiluous Millars. 



THE DECIDUOUS TEETH IN DETAIL. 



2 33 



lingual is compressed in the opposite direction. The apical ends of the 
roots are much separated from one another, the triangle which these 
points form being almost twice the size of the triangle formed by the base 
of the roots. The apical ends are usually provided with a central curve. 



Upper Second Molar. 



c3 
U 



Twenty-two 
Months Old 



One Year Old 



Six Months 
after Birth 

Birth 




Eight Years 



Ten Years 



Eleven Years 



a 



Fig. 175. 

Calcification Begins, between the Fifth and Sixth Fetal Months. 

Calcification Completed, Twentieth to Twenty-second Month after Birth. 
F.rupted; Eighteenth to Twenty-fourth Month after Birth. 
Decalcification Begins, Seventh to Eighth Year. 

Shedding Process Takes Place, Eleventh to Twelfth Year. 
Average Length of Crown, .22. 

Average Length of Root, .46. 

Average Length over All, .68. 



The most remarkable feature about the crown of this tooth is its close 
resemblance to the crown of the upper permanent first molar. The 
various surfaces are almost identical, the de- 
velopmental process, and consequently the cusp- 
formation, is the same, the marginal and other 
ridges common to the occlusal surface corre- 
spond, and both the central and distal fossae are 
present, together with the various developmental 
grooves. A description of the crown will, there- 
fore, be unnecessary; suffice it to say that it is 
much smaller in every direction and is some- 
what more constricted at the neck. The roots 
are the same in name and number as those of 
the first permanent molar, but they are more 
widely separated at their apical extremities. In general form they are 
smaller than those of the upper first deciduous molar. 




Fig. 176. 
ond M o 
Surface. 



—Upper Sec- 
a r, Buccal 



234 



ANATOMY. 



THE LOWER DECIDUOUS TEETH. 

A description in detail of the lower incisors and cuspids would practi- 
cally be a repetition of that given of the corresponding upper teeth, and 
for that reason will be passed with a limited reference to each. The 
lower molars being in many respects unlike the upper, they will require 
a separate description. 

Lower Central Incisor (Fig. 177). 

Calcification 1 Begins, about the Fourth Fetal Month. 

Calcification Completed, Sixteenth to Eighteenth Month after Birth. 
Erupted, Sixth to Eighth Month after Birth. 
Decalcification Begins, about the Fourth Year. 

Shedding Process Takes Place, about the Seventh Yeas. 
Average Length of Crown, .19 

Average Length of Root, .35. 

Average Length over All, .54. 




Fig. 177 — 
Lower Central 
Incisor, LaMal 
Surface. 



This is the smallest of the lower teeth, in this 
respect being at variance to the upper central, which 
is larger than the lateral. The mesiodistal diameter 
of the crown is but little less than that from the 
cutting-edge to the cervical line. The mesial and 
distal angles are similar, both being pointed and 
square. The cervical ridge is quite pronounced and 
the neck much constricted. 

The root is usually straight and tapers gradually 
from base to apex. It is broader on the labial than 
on the lingual side, and the mesial and distal sides 
are but little flattened. 



Lower Lateral Incisor (Fig. 178). 

Calcification Begins, about the Fourth Fetal Month. 

Calcification Completed, Twelfth to Fourteenth Month after Birth. 
Erupted, Seventh to Ninth Month after Birth. 
Decalcification Begins, about the Fifth Year. 

Shedding Process Takes Place about the Eighth Year. 
Average Length of Crown, .19. 

Average Length of Root, .39. 

Average Length over All, .58. 

This tooth is larger than the central incisor, and closely resembles the 
upper lateral both in size and form. The crown is more rounded in its 



THE DECIDUOUS TEETH IN DETAIL. 



2 35 



nature than that of the central, forming a greater general convexity to 
the labial surface, and less concavity to the lingual. The mesial angle of 
the crown is fairly well defined, while the distal is 
usually much rounded by a long, circular sweep of 
the cutting-edge to meet the distal surface. 

The mesial surface of the crown is flattened and 
somewhat prominent at the angle, while the distal 
surface is strongly convex. The labial grooves are 
but slightly visible, while the corresponding lingual 
grooves are quite pronounced. The neck of the tooth 
is even more marked than that of the lower central 
incisor. The root is long and tapering, slightly flat- 
tened from mesial to distal, with a decided longitudinal 
groove on both the mesial and distal sides. The 
labial and lingual sides are rounded, and there is an 
inclination to crookedness, which is usually from mesial to distal. 




Fig. 178. — 
Lower Lateral 
Incisor, Labial 
Surface. 



Lower Cuspid (Fig. 179). 



Calcification Begins, about the Fifth Fetal Month. 

Calcification Completed, about Two Years after Birth. 

Erupted, Seventeenth to Eighteenth Month after Birth. 
Decalcification Begins, about the Ninth Year. 

Shedding Process Takes Place, about the Twelfth Year. 
Average Length of Crown, .23. 

Average Length of Root, .45. 

Average Length over All, .68. 




Fig. 179. — 
Lower Cuspid, 
Labial Surface. 



The principal variations between this tooth and 
the upper cuspid are observed in the diminished 
mesiodistal measurement of the crown, together with 
it being somewhat less angular in outline. The 
ridges and grooves common to the various surfaces 
are not so marked as those of the upper cuspid, re- 
sulting in a smoothly formed crown throughout. The 
root is larger in proportion to the size of the crown 
than that of its upper opponent, thus producing a 
neck much less constricted. It is usually straight, or 
provided with a slight distal inclination near its apical 
extremity, and much flattened from mesial to distal, 
these two sides converging to the lingual, forming a 
rounded triangular outline. 




236 ANATOMY. 

Lower First Molar (Fig. 180). 

Calcification Begins about the Fifth Fetal Month. 

Calcification Completed, Eighteenth to Twentieth Month after Birth. 
Erupted, Fourteenth to Fifteenth Month after Birth. 
Decalcification Begins, Sixth to Seventh Year. 

Shedding Process Begins, about the Tenth Year. 
Average Length of Crown, .24. 
Average Length of Root, .38. 

Average Length over All, .62. 

Upon making an examination of the occlusal surface of this tooth it 
will be observed that the crown is made up of four irregularly formed lobes, 
separated from one another by four well-defined grooves. Each lobe is 

provided with a cusp, more or less prominently 
developed. Between the various cusps are two 
fossae — one occupying the distal two-thirds of the 
surface (the distal fossa) and the other the re- 
maining or mesial third (the mesial fossa). 
The outline of this surface, which represents 
the contour of the crown in general, is that of 
an oblong square, with its angles more or less 
Fig. 180.— Lower First rounded, and having a slight variation in its 

Molar, Buccal Surface. _ . , , 

parallel lines. Each lobe denotes a separate 
center of calcification, and the four grooves the lines of union between 
the various parts. 

The mesiobuccal lobe is somewhat irregular in contour and is fre- 
quently the largest of the four. It assists in forming the mesiobuccal 
angle of the crown and the greater part of the mesial fossa. Descending 
from this cusp to the lingual is a pronounced triangular ridge, which is 
made continuous by uniting with a similar ridge from the corresponding 
lingual cusp. By this union a transverse ridge is established, separating 
the mesial from the distal fossa. The central boundary of this lobe 
is formed by the mesial groove, which arises from the distal fossa, passes 
over the transverse ridge to the mesial fossa, from which it continues to 
the lingual, and by the buccal groove, which branches off from the 
mesial somewhat to the distal of the transverse ridge, passing over the 
buccomarginal ridge to the buccal surface. 

The Distobuccal Lobe. — This cusp is generally smaller than the 
mesiobuccal, and is more pointed and more regular in outline. It assists 
in forming the distobuccal angle of the crown, and by its central incline 
forms about one-third of the distal fossa. Its boundaries are formed by 
the buccal, mesial and distal grooves, the latter beginning in the distal fossa, 
and passing over the distomarginal ridge to the distal surface. 



THE DECIDUOUS TEETH IN DETAIL. 237 

The Mesiolingual Lobe. — In the recently erupted tooth the summit 
of this cusp is long and pointed, and frequently remains the most pro- 
nounced of the four. It is triangular in outline, and, as above referred to, 
furnished a triangular ridge, which, by uniting with a like ridge from the 
mesiobuccal cusp, forms the transverse ridge. By its central incline it 
assists in forming the mesial fossa. Its boundaries are formed by the 
mesial groove and the lingual groove, the latter arising near the center of the 
distal fossa, passing to, and sometimes crossing, the linguomarginal ridge. 

The Distolingual Lobe. — This is usually the smallest of the four. 
It is inclined to be rounded, rather than angular, and in some instances 
is poorly developed. It assists in forming the distolingual angle of the 
crown, as well as a portion of the distal fossa. Its central boundaries are 
formed by the lingual and distal grooves. 

The marginal ridges of the surface are abruptly but irregularly 
formed, ascending and descending the various cusps in a manner similar 
to those previously described. 

The Buccal Surface of the Crown (Fig. 180). — This surface is 
smooth and generally convex, with a mesiodistal measurement about 
twice as great as that from the cervical line to the occlusal margin. 
The surface is most prominent over its cervical third, forming a well- 
rounded and bold cervical ridge, a feature strongly characteristic of this 
tooth. The distal center of the surface is broken by the buccal groove, 
which usually ends near the center in a shallow depression or pit. 

The Lingual Surface of the Crown. — This surface is much less 
extensive than the buccal. It is smooth and convex throughout, 
and is broken near its distal center by the lingual groove, which gradually 
disappears as it passes rootward. The cervical ridge is not so prominent 
as that of the buccal surface. 

The Mesial and Distal Surfaces of the Crown. — These are slightly 
convex in every direction, the former passing, by a gradual sweep, into 
the lingual surface, destroying the angularity of the crown at this 
point, while the latter passes more abruptly, forming an acute angle. 
These surfaces are both prominent near the occlusal margin, making the 
point of contact with adjoining teeth near that surface. The bold cer- 
vical ridge of the buccal surface is discontinued or greatly diminished 
upon these surfaces, both of which are inclined to pass very gradually 
into the base of the roots. 

The roots of this tooth are the same in name, position, and number 
as those of the lower permanent molars. They are much flattened from 
mesial 'to distal, the center of their flattened sides being further com- 



238 ANATOMY. 

pressed by a deep longitudinal groove, which extends from the base to 
the apex of each root. In passing from the base of the roots to their 
apices they become more widely separated, until these extremities are 
much wider apart, proportionately, than those of the permanent molars. 

Lower Second Molar (Fig. 181). 

Calcification Begins, between the Fifth and Sixth Fetal Months. 

Calcification Completed, Twentieth to Twenty-second Month after Birth. 
Frtjpted, Eighteenth to Twenty-fourth Month after Birth. 
Decalcification Begins, Seventh to Eighth Yeak. 

Shedding Process Takes Place, Eleventh to Twelfth Year. 
Average Length of Crown, .21. 
Average Length of Root, .44. 

Average Length over All, .65. 

The anatomy of this tooth being almost identical to that of the 
lower first permanent molar, it will be unnecessary to enter into a descrip- 
tion in detail. The lobes, and consequently the 
grooves, are the same in position, name, and 
number, and a similar developmental process is 
recorded. The tooth is not characterized by a 
prominent cervical ridge, such as is found upon 
the lower first molar, the crown passing very 
gradually into the root-base with a neck moder- 
ately constricted. 

THE PULP-CHAMBERS AND CANALS OF 

" ~d THE DECIDUOUS TEETH. 

Molar, Buccal Surface. A few general re marks in reference to these 

cavities, in connection with the information to be derived from the 
accompanying chart and its annexed description, will sufficiently instruct 
the reader, without the necessity of treating each tooth individually. 
The pulp-chambers and canals, like those of the permanent organs, 
assume the form of the external contour of the tooth, the crown of the 
tooth being provided with a central cavity, the pulp-chamber, partaking 
of outlines closely resembling those of the crown, while the root is 
traversed by the pulp-canal, likewise conforming to the shape of the 
root. One very important distinction between the pulp-chambers and 
canals of the deciduous teeth and those of the permanent organs is that 
the former are proportionately larger. It must also be noted that the 
apical foramina in these teeth are so transitory in their nature that there 
remains but a very brief period during which the canals may be said 
to be fully formed. It will be recalled that in a very short time after 
the roots have become completely calcified, decalcification begins, and 




THE DECIDUOUS TEETH IN DETAIL. 



2 39 



this process of degeneracy, beginning at the apical extremities of the 
roots, very early destroys the foramina, which have in a measure served as 
a protection to the surrounding parts during operations upon the canals. 
With the canals proportionately larger than those which occupy the 
roots of the permanent teeth, the foramina during their very limited 
existence are also much larger, and much more readily penetrated. 
With these ever-changing conditions in the pulp-canals of the deciduous 
teeth, it is of importance that a definite knowledge of what takes place 
should be acquired, and it is for this purpose that the accompanying 
chart has been prepared (Fig. 182). 




<5i 



O 10 

h-ts 



3 § 



C l—l 

■a 3 

c3 O 



c 
c o 

d n, 

~ Oh 

a v 



I u 



00 



uoiaBog!3^3 



notjEogiojEoaQ 



CHAPTER XII. 

Development of the Teeth.— The Dental Germs, Enamel Organ, 
and Dentin Organ; the Dental Follicle; Calcification and 
Eruption. 

DEVELOPMENT OF THE TEETH. 



Oral Epithe 
Hum 



Infant Layer 
of Cells 



Stellate Retic 

ulum 



Ameloblasts 








Dentin Papilla 



Fig. 183. — Developing Tooth-germ. X 300. 

In order that the student may obtain a general idea of the structural 
changes which take place at a very early period, and which eventually result 
in the formation of the teeth, the genesis of the subject will be briefly re- 
ferred to. Preparation for the development of the teeth takes place as early 
as the middle of the second fetal month, this preparatory alteration in the 

240 



DEVELOPMENT OF THE TEETH. 241 

tissues beginning before the process of ossification in the bony structures 
which finally surround and give support to the organs. At this early 
period there will be found following the line of the future alveolar ridge 
a slight heaping up of the surface epithelium, while immediately beneath 
this proliferation of cells there appears a dipping in of the deep epithelial 
layer in the direction of the future alveolar walls. This epithelial reflec- 
tion is known as the epithelial band or tooth-band. This so-called tooth- 
band is not, as might be supposed, a special inflection for each tooth- 
germ, but is continuous from one end of the future jaw to the other. 
It must be remembered that at this time the outline of the jaws has not 
been established, and the tooth-band, although not generally considered 
as essential to the developmental process, is principally instrumental 
in directing the position of the dental organs. The position and form 
of this epithelial band may best be studied in vertical transverse section. 
When first making its appearance it is somewhat broad and shallow, 
but as it passes more deeply into the parts it partakes of the outline of the 
letter V with its open end directed toward the surface. In penetrat- 
ing the subjacent tissue, the free extremity of the band is inclined to the 
lingual, its external surface is slightly convex, and its internal surface 
correspondingly concave. Histologically considered, the tooth-band is 
composed of elements similar to those which serve to make up the 
epithelial layer of the oral mucous membrane. After the tooth-band 
has assumed certain proportions, there appears on its inner or concave 
surface a thin membranous plate, which is likewise a continuous structure, 
extending the full length of the epithelial band. 

This lamina does not spring from the free margin of the tooth-band, 
but is given off at a point about midway between this border and the base 
of the band. The structure of this secondary band is so similar to that 
of the primary one that it should be considered as an inflexion from it 
rather than a new structure. We find them between the seventh and 
eighth week, the maxillary regions giving place to two bow-shaped 
bands (one for each jaw), each of which is preparing to throw out from 
its secondary lamina ten little buds, which soon develop into the germs 
for the twenty deciduous teeth. When these buds make their appear- 
ance they are simple, rounded bodies, placed somewhat closely together, 
but they do not long retain this simple form. The first change which 
takes place is one in which they appear to lengthen out into slender cords, 
the extremities of which soon begin to extend laterally, and the primitive 
enamel organ is formed. Accompanying this lateral extension of the 
periphery, a bell-shaped outline is assumed, which phenomenon is 
16 



242 



ANATOMY. 



rapidly increased by a specialization of the surrounding cellular tissue 
forcing into the concavity. 

This bell-shaped proliferation of cells, given off directly from the 
tooth-band, to which it continues for a time attached, together with the 
specialized connective tissue crowding into its concavity, constitute the 
tooth-germs, the former being the enamel organ, and the latter the dentin 
organ. It will, therefore, be seen that the enamel is dependent upon 
the oral epithelium for its development (ecdermic), while the dentin 
springs from an entirely different source — the connective tissue of the jaw 



Epithelium of 
Upper Jaw 



Infant Layer 
of Cells 




Epithelium of Lower Jaw 



Neck of Cord 



Stellate Retic- 
ulum 



Ameoblasts 



Dentin Papilla 



Forming Be 



X 



Fig. 1S4. — Developing Tooth-germ, Twelfth Week. X 4c. 



(endermic). The enamel organ rapidly undergoes a cellular transfor- 
mation: its concavity is increased, and the bell-shaped outline more 
strongly defined. Accompanying this change in form it gradually recedes 
from the surface, and its connection with the tooth-band becomes less 
secure. The connective-tissue cells, which have been rapidly filling in 
the concavity of the enamel organ, are also preparing to take upon them- 
selves a special function, that of the formation of the dentin. Up to this 
period (tenth week) the enamel and dentin germs are not definitely 
separated from the surrounding cellular structure, but now a gradual 
transformation takes place, whereby the tooth-germs become enveloped 
in a sac-like covering — the dental follicle. 



DEVELOPMENT OF THE TEETH. 243 

Enamel Organ (Figs. 183, 184, and 185). — This portion of the 
tooth-germ, as previously stated, is derived from the concave or lingual 
surface of the tooth-band, which in turn is derived from the surface 
epithelium. From the free extremity of its slender cord-like attachment 
it spreads out and forms a hood-like covering to the dentin germ. The 
surface of the organ contiguous to the dentin germ, or dentin papilla, as 
it is frequently called, is concave in the direction of the oral surface, 
being thickest over the center of its concavity, thinning down as its per- 
iphery is approached. Externally, the enamel organ is covered by an 
epithelial layer, which is reflected upon its inner surface or that in contact 
with the dentin papilla. These two layers are named according to their 
location, the external and internal epithelium of the enamel organ. Placed 
between these two layers, and constituting the bulk of the organ, are 
numerous stellate bodies which penetrate a layer of rounded cells, the 
stratum intermedium, and finally reach the internal epithelial layer 
known as the enamel cells or ameloblasls. It is from this internal layer 
of epithelial cells that the enamel is calcified, and they are, therefore, 
the essential cells of the enamel organ. In the fully developed enamel 
organ, there are to be found, therefore, four distinct layers of cells, the 
external epithelium, stellate reticulum, stratum intermedium, and internal 
epithelium, or ameloblasls. As its name implies, the function of the 
enamel organ is principally that of enamel calcification, but in the opinion 
of many writers its primary activity is that of molding the tooth-form 
as represented by the dentin papilla, and it is not until this latter organ 
has assumed the form and extent of the future tooth-crown that dentin 
calcification begins. 

The life of the enamel organ may properly be considered as begin- 
ning when the bulbous extremity of the specialized cells given off from 
the lingual face of the tooth-band become invaginated, and from this by 
a rapid proliferation of its cells it passes on by successive stages assum- 
ing the various forms common to it. This proliferation and differentia- 
tion of cells continues up to the time of beginning of calcification, but 
with the advent of this phenomenon certain parts of the organ begin to 
degenerate. This degeneration may or may not be classed as an atrophy 
of the cells interested, but the fact that a new tissue is generating, and 
gradually occupying the space previously taken up by the formative cells, 
calls forth a demand for the removal of the latter by the former. The 
cells which first undergo this change are those of the internal epithelium 
and stratum intermedium, the individuality of these two layers evidently 
being kept up by migratory cells from the stellate reticulum. It is argued 



244 



ANATOMY. 



by some writers that the external epithelium begins to atrophy at this 
period; by others this change is not recorded until the enamel cuticle has 
been deposited to effectually seal the young tissue and protect it until well 
desiccated. While there appears to be a decided disposition upon the 
part of this outer layer of cells to change, they do not disappear, and the 
alteration is not one which affects the shape of the cells, for they remain 
flattened or prismatic with their long axis placed parallel with the anlage 
of the crown. The stellate cells making up the bulk of the organ are, 




FlG. 185. — Developing Tooth-germs, Enamel Organ, and Dentin Papilla. 

in common with those which inclose them, continually undergoing a 
degenerative change, at least this is true of those cells closely associated 
with the stratum intermedium, for in this location they rapidly proliferate, 
shed their many processes, and gradually take on the characteristics 
common to this layer of which they eventually become a part. A careful 
examination of no less than one hundred enamel organs in all stages of 
development, and by section cut transversely, obliquely, longitudinally, 
etc., fully justifies the statement that the real life of the enamel organ 
begins as previously stated, and continues until the structural arrangement 
of the enamel is completed. 



DEVELOPMENT OF THE TEETH. 



245 



The question of form in the enamel organ — that is, its external epithe- 
lium — is one which may be advantageously used in the consideration of 
the life and function of its different cell layers. It has been said that the 
apparently extravagant area taken up by the enamel organ subserves 
the purpose of reserving space for the growing tooth-crown, but there are 
many reasons why this theory cannot be accepted. In the first place, 
the extent of the organ or the space existing between the dentin papilla 
and the outer enamel epithelium does not in very many instances corre- 




Fig. 186. — Developing Tooth-germs. Longitudinal Sections from Buccal to Lingual. 

spond to the bulk of enamel when this tissue is completed at a given point. 
In the developed tooth we find the enamel thickest over the cutting-edges 
of the anterior teeth and about the summits of the cusps of the cuspidate 
teeth, while these same parts are represented during the cellular stage of 
development by the external layer of cells closely associated with the 
surface of the papilla. 

Again, in very many instances the outline of the tooth is definitely 
represented by the cells making up the dentin papilla (Figs. 185 and 186), 
but the surrounding epithelial cells are characterized by an unbroken 



246 



ANATOMY. 



semicircular margin describing the extent and form of the enamel organ. 
Exception may be taken to this hypothesis from the standpoint of genera- 
tive changes, and these in a great measure have much to do with the 
relative outlines assumed by the two organs, but by studying very many 
sections representing nearly every stage of the process, and all of them in a 
measure showing the same characteristics, nothing but a definite opinion 
can result. 

Of the many changes in general form which the enamel organ 
undergoes, none are so pronounced and positive in character as those 




Fig. 187. — Developing Tooth-germs. Longitudinal Section from Mesial to Distal. 

described by the inner tunic, and first recorded when the bulbous end of 
the specialized cells becomes invaginated by the mesodermic connective- 
tissue cells forcing themselves into it. This is an alteration which is 
gradual and continuous up to the time of beginning of calcification, and 
while the cells forming the dentin papilla are generally accorded the power 
of "pushing" or "forcing" their way into those derived from the epi- 
blast, the latter has always been recognized as having a controlling influ- 
ence over the former. In this connection a reasonable doubt presents 



DEVELOPMENT OF THE TEETH. 



247 



6 



itself covering the theory so long accepted that the early function of the 
enamel organ is one which in a measure superintends the contouring f 
the tooth-crown as first represented in the dentin papilla. When the 
character of the two embryonal tissues making up the two germs is Com- 
pared, we find the dentin germ possessing all the characteristics favor- 
able to a rapid proliferation of its cells resulting in a highly vascular 
compact tissue. On the other hand, the bulk of the enamel organ j s a 
gelatinous-like mass, one that would readily succumb to the pre ssure 




Fig. 188. — Developing Tooth-germs in Transverse Section. 
A, Stellate reticulum. B, Papilla. C, Cartilage cells. 



exerted by those active connective-tissue cells within its borders. When 
thus considered, the evidence is almost sufficiently convincing to reverse 
the generally accepted theory, placing the general form of the enamel 
organ as subservient to the dentin papilla. 

Figures 185, 186, and 187 illustrate some of the variations common to 
the general form of the enamel organ, and afford a good idea of the rela- 
tionship existing between the enamel organ and the dentin papilla, in teeth 
both of the simple and complex class. These were taken from sections 



248 



ANATOMY. 



which represent a period just prior to the generation of the ameloblasts 
and odontoblasts, at which time the external and internal epithelial 
layers of the enamel organ most closely resemble one another, in general 
outline. It is from this aspect and from sections cut longitudinally that 
most of the information given by the older writers has been derived. 
Few attempts have been made to show this organ in sections transverse 
to the long axis of the tooth anlage. In figure 188 the germs of two teeth 
are shown by a section made in this direction. One striking feature here 



a 



A* 



$**. 




3. 



Fig 

A. Inner tunic of enamel organ. H, Cells <>f dentin papilla. 

illustrated is the relationship existing between the inner and the outer 
tunic of the enamel organ, and attention is called to the apparent coales- 
cence of these two layers at those points which represent the mesial and 
distal surfaces of the developing crowns. This condition is apparently 
brought about by the cartilage cells forcing the peripheral cells of the 
enamel organ into direct contact with the inner tunic completely obliterat- 
ing the stellate reticulum in these localities. As a result of this lateral 
pressure the outer epithelial cells representing the labial and lingual 



DEVELOPMENT OF THE TEETH. 



249 



surfaces have become widely separated, but with no perceptible alter- 
ation in the character of the cells composing the stellate reticulum. The 
relationship existing between the tooth-germs and the surrounding parts 
is one that will continue throughout the generation of the organs, and 
makes questionable the theory that the stellate reticulum performs 
the function of reinforcing or providing the ameloblasts with nutrient or 
calcific material. If these same germs were examined, as they usually 
are, in longitudinal section (see Figs. 185, 186, and 187), the investigator 





S>f 



a. 



|#. 



Fig. 190. 
A, Cells of Dentin papilla. B, Elongated cells of inner tunic. 

would at once arrive at the conclusion that there was an equal distribu- 
tion of the stellate cells about all sides of the dentin papilla. In their 
very early life they apparently establish an equal bulk about all sides of 
the dentin germ, but with the preparation for the growth of the alveolar 
walls they may assume the proportions shown in figure 188. At a period 
corresponding to the complete envelopment of the dental germs by the 
dental follicle the development of the buccal and lingual walls is well 
under way, but as yet no provision has been made for the septa between 



250 ANATOMY. 

the teeth, and it is undoubtedly to the approach of this latter phenomenon 
that a definite lateral pressure is brought to bear upon the approaching 
of the follicles. 

Now let us pass to a consideration of some of the characteristics of 
the various cell layers composing the enamel organ. These are desig- 
nated according to their location, so far as three of the four layers are 
concerned; in fact, the remaining cells, or those which receive their name 
from their form, can scarcely be classified as a distinct layer, these stel- 
late cells not being of uniform thickness in all parts of the organ. The 
first layer of cells, or those making up the inner tunic, will be traced from 
their primary spherical condition to their final generation into ameloblasts. 
Figure 189 shows the character of these cells at a very early period, corre- 
sponding to the sixteenth week in the human fetus. They are for the 
most part spherical or slightly oblong multinucleated cells, and are more 
or less closely associated. They partake very much of the nature of the 
connective-tissue cells surrounding them, being differentiated from these 
principally by a transparent zone not unlike the specialized matrix im- 
mediately surrounding cartilage cells. About the first change recorded 
in these cells (see Fig. 190) is one in which they become markedly elon- 
gated or cylindrical, but during this process of differentiation some of the 
cells apparently recede, while others advance in the direction of the 
papilla, lining up in a single layer to become the early enamel cells, the 
cells which have been thus forced to the rear subsequently developing 
into ameloblasts as the older cells atrophy. At this period the stratum 
intermedium also asserts itself in the form of a distinct layer of rounded 
cells, to be described later on. When first observed, these cylindrical 
cells are devoid of processes, but are provided with rounded extremities, 
with little or no variation between the end directed toward the papilla 
and that looking in the opposite direction. This form is one which 
persists in all of the cells included in this layer until a definite body of 
cells is formed contiguous to the dentin papilla, these latter cells becoming 
more markedly elongated and further differentiated by the addition of 
processes, while the remaining cells, or those nearest the stratum inter- 
medium, continue for a time unchanged. The next alteration in the 
character of the inner tunic is one well illustrated in figure 190, in which 
the body of the generating ameloblastic cells rapidly recede from the surface 
of the papilla, while the elongating processes reach out to this latter point, 
all of this occurring before the appearance of the odontoblasts. Soon 
after this latter change in the cells of this layer the odontoblasts are 
developed and form the periphery of the dentinal tissue. All of these 



DEVELOPMENT OF THE TEETH. 



25 1 



changes are of course first recorded about the free extremity of the tooth- 
crown, becoming less noticeable as the union of the outer and inner tunics 
is approached. 

A study into the special characteristics of the fully developed amelo- 
blasts shows that these active cells are the result of a gradual change in 
the character of the columnar epithelia common to both the external and 
internal epithelial layers in the primitive enamel organ. 




Fig. 191. 

A, Stellate reticulum. B, Stratum intermedium. C, Ameloblasts. D, Forming enamel. 
E, Calcified dentin. F, Odontoblasts. 

Next in importance to the internal epithelial layer are those closely 
associated cells making up the stratum intermedium (Fig. 192). Primarily 
oval or spheroidal in form, we find these cells gradually assuming a 
columnar outline and occupying a position parallel to the long axis of the 
crown. It may be said that the general character of these cells is inter- 
mediate between those destined to become the proper enamel cells and 



252 



ANATOMY. 



those stellate cells making up the bulk of the organ. There appears to 
be much confusion, at least considerable doubt, in regard to the office of 
the cells of the stratum intermedium. It is most likely that these cells are 
not only intermediate in character, but are also intermediate in func- 
tion to those cells upon either side of them, recruiting the ameloblastsas 
they fall, while in turn they themselves are supplied with nutriment from 
the enamel pulp or stellate reticulum. No stronger proof that these 
cells are secondary in importance to the ameloblasts need be mentioned 




Fig. 192. 
\, Generating ameloblasts. B, Rounded cells of stratum intermedium. C, Stellate reticulum, 

than reference to the fact that they are always more generously supplied 
to those parts about to undergo calcification. Nor is their increase in 
numbers the only reason for believing that they are thus employed, for 
at the same time those cells most closely associated with the developing 
ameloblasts take upon themselves a decided change in outline. This 
alteration may be brought about by the conditions which influence the 
shapes of all cells, i.e., by the pressure of surrounding cells or by 
their preparation for functional activity, or both. Some of the older 



DEVELOPMENT OF THE TEETH. 253 

writers speak of the cells of this layer as being branched, and in this 
way closely resembling those of the stellate reticulum. By strong am- 
plification it is somewhat difficult to distinguish between the two 
layers, but most certainly if there are branched cells present they are con- 
fined to the intermediate zone, and should properly be classed with 
those of the stellate reticulum. 

The cells of this layer do not long remain columnar with a general 
direction at right angles to the forming ameloblasts, but as these latter 
cells they become spheroidal and extremely closely associated in the deeper 
portion of the layer; in fact, cells corresponding to these in general appear- 
ance may be found in connection with the fully developed ameloblasts, 
being observed to best advantage by the aid of a high power objective 
and a full flood of light from a powerful sub-stage condenser. The 
cells thus found appear to be distributed at regular intervals about the 
ameloblastic layer, and are so closely allied to the cells of the stratum 
intermedium that they may be considered as migratory cells from this 
layer. In the earlier stages there appears to be no definite line of demar- 
cation between the cells of the inner tunic, and those composing the 
stratum intermedium, but soon after the establishment of the amelo- 
blasts the two layers are strongly differentiated by the interposition 
of a highly transparent membrane covering the outer extremities of the 
ameloblasts. After they are thus definitely separated from the enamel- 
forming cells, a most radical change takes place in their character; they be- 
come markedly elongated, and by anastomosing form a series of continuous 
chain-like belts about the ameloblastic layer, the number and further char- 
acter of which are dependent upon the extent to which the ameloblasts 
have performed their function. If at any time there is a similarity between 
the cells of the stratum intermedium and the stellate reticulum it is at 
this period, because the former cells begin to lose their individuality, 
although under low power they still appear as a distinct layer (Fig. 191) . 

There is probably no body of cells directly interested in the develop- 
ment of the tooth tissues so widely discussed as those making up the so- 
called stellate reticulum, and while the chief basis for argument has been 
with reference to their function, the general character and form of the 
cells have received but little consideration. Ever since the first description 
of this portion of the enamel organ the cells therein have been charac- 
terized as "star-shaped," and while this stellate form is the most common, 
it is by no means a universal condition. The form of the cells in common 
with the other cells composing the organ appears to be much influenced by 
the position which they occupy, and by the age of the organ, those cells in 



254 ANATOMY. 

the region of the inner tunic partaking of the globular form characteristic 
of this layer (see C, Fig. 192), while those closely associated with the outer 
tunic are inclined to be columnar or somewhat elongated. While the 
bodies of the cells in these respective locations are more or less influenced 
by their environments, they still retain to a certain extent the stellate 
feature by their many processes. But it is in the center of this myxom- 
atous epithelial product that the most perfect stellate cells are located. 
We find, therefore, where this part of the organ is of the greatest width, 
that the true stellate cells are the most numerous, while at the summit of 
the crown and at the base of the organ, at both of which points the outer 
and inner epithelial layers are closely associated, the star-shaped cells 
are almost wanting. In the study of this layer very much depends upon 
the thickness of the section, only the thinnest possible sections affording 
an opportunity for a correct conception. This is, of course, true of all 
parts of the organ, but the peculiar character of the stellate reticulum 
makes it especially necessary that great care be bestowed upon the prep- 
aration of the section. In transverse section the cells present no charac- 
teristic differences from those shown when the section is made longitudi- 
nally. One very pronounced feature about the cells of the stellate retic- 
ulum is the granular appearance of their protoplasm, resembling very 
closely the flattened squamous cells from the epithelium of the mouth, 
and it is no doubt this special feature which furnishes the ground for the 
opinion of many writers that it is a peculiarly modified epithelium. One 
peculiarity in connection with this tissue which iscontrary to the generally 
accepted character of epithelial cells is the abundance of intercellular 
cement substance; but when the many minute spines or processes 
are considered as a part of the individual cell, the proportionate 
quantity of cellular and intercellular substance is somewhat decreased. 
The connecting processes are quite similar to those described by Stohr 
as connecting bridges of protoplasm, while the cells themselves may be 
otherwise described as prickle-cells. The change in the form of the cells 
of this layer is not due to the presence of neighboring cells, as in the case 
with most epithelial cells, but, being soft and extremely plastic, it is more 
than likely that their form is strongly influenced by the tension of their 
connecting filaments. One of the most marked alterations in the 
general character of this part of the enamel organ is that which takes 
place at a time corresponding to the beginning of amelification, and is no 
doubt attributable to this phenomena. The cells which up to this period 
have remained widely separated now become more closely associated, 
not so much by a change of position as by what appears to be an increase 



DEVELOPMENT OF THE TEETH. 255 

in the size of the cell body with a corresponding decrease in the length 
of the anastomosing processes. 

It is a fact admitted by most histologists that the peculiar star-like 
nature of the cells of the stellate reticulum is one principally brought 
about by postmortem changes, and that in reality they are polygonal cells 
tilling up a greater part of the tissue with but little intercellular sub- 
stance. That some shrinkage and distortion does take place may be 
proved by the examination of a section which has accidentally or other- 
wise become for a moment dry during its preparation, in which case 
little can be seen but the connecting processes, and even these are much 
shrunken. All the cells contained within the organ are more or less 
affected by this procedure, but none of them exhibit such a marked 
change in the outline as those of the stellate reticulum. 

The layer of cells which is usually considered of least impor- 
tance is that which makes up the outer tunic. In the young enamel 
organ the cells partake very much of the nature of these forming the inner 
tunic, but the older the organ becomes, the more dissimilar are the two 
layers. Primarily this layer is constructed of a single row of elongated 
cells, placed with remarkable regularity, upon the inner side of which are 
a number of similarly formed cells variously disposed, but with a common 
direction at right angles to those previously referred to. Like the inter- 
nal epithelial layer, the cells of the outer tunic partake more or less of 
the nature of the stellate cells in passing from the single row of well- 
defined cells toward the stellate reticulum. While in the beginning 
the external epithelial layer is strongly differentiated from the sur- 
rounding cells, this is of but short duration. The atrophy of 
this layer begins with the appearance of the fully developed amelo- 
blasts, by which the regular arrangement of the cells is greatly disturbed 
by an apparent breaking up of the entire layer. Many reputable writers 
claim that the external epithelial layer is of little or no interest, save, as 
Tomes puts it, "as a matter of controversy." This admission upon the 
part of so eminent an authority practically opens up a new field for re- 
search, especially so when we consider that various other writers (Wald- 
eyer, Kolliger, and Magitot)have expressed conflicting opinions in regard 
to it. After carefully following the changes which occur in this layer 
from its earliest inception up to an advanced stage of calcification, it 
would appear that while marked changes occur in the character of the 
individual cells as well as in the general appearance of the layer, it is 
nevertheless persistive, and in some way is essential to the process of 
amelification, even to a more marked degree than are the cells of the 



256 ANATOMY. 

stellate reticulum. One important reason for this belief is based upon 
the fact that in the cuspidate teeth there appears at a time corresponding 
to the beginning of amelification, a decided disposition in the cells of 
this layer to dip down and completely divide the stellate reticulum be- 
tween the forming cusps. That this alteration is one instrumental or 
essential to the calcifying process receives additional proof by referring 
to figure 185, which shows the fully developed enamel organ with the 

Dental 
Ridge, 
Upper 
^ Jaw 



Oral Cavity 




Tongue — 

^g Developing 
\ Tooth-germ 



Fig. 193. — Section through the Floor of the Mouth of Human Fmbryo. 
Twelfth Week. X 30. 

exception of the actual appearance of the ameloblasts, and the lack 
of any attempt upon the part of the external epithelial layer to penetrate 
between the cusps. 

Dentin Organ (Fig. 184). — This part of the tooth-germ, formed from 
the connective tissue of the primitive jaw, occupies the concavity of the 
enamel organ, and at an early period begins to assume the form of the 
future tooth-crown. Thus, primarily, the papillae for the incisors will 
have their cutting-edges outlined by three small lobes, each of which 
represents a separate point of calcification, while the papillae for the molars 
will be molded according to the number of cusps of the future tooth, a 
small tubercle making its appearance for each cusp. In its inception the 
dentin papilla is composed of cellular tissue identical with that of the sur- 



DEVELOPMENT OF THE TEETH. 257 

rounding parts. The growth of the papilla is in the direction of the surface ; 
at the same time the enamel organ forces itself more deeply into the sub- 
stance of the parts, not only overhanging the coronal extremity of the 
papilla, but extending about and inclosing its lateral walls. Accompany- 
ing the growth of the papilla is a rapid change in its structure, becoming 
more vascular throughout, and its peripheral cells, differentiating, form 
the essential dentin cells — the odontoblasts. This layer of cells is in 
close relation to the enamel cells of the enamel organ, the combined 
activity of the two finally resulting in the calcification of the tooth-crown. 
The dentin papilla, which eventually becomes the tooth-pulp, decreases 
in size as calcification proceeds to the dentin, all additions to the calci- 
fying surface taking place from within; while the enamel organ may be 
said to increase in size, the calcific action in the enamel progressing from 
within outward. 

The Cells of the Dentin Papilla. — In the early life of the dentin 
germ, the cells are all simple embryonal connective-tissue cells. After 
differentiation takes place they are widely scattered and are of four 
varieties; spindle-shaped, round, stellate, and the elongated or club- 
shaped odontoblasts. None of these are constant in location except the 
layer of odontoblasts, which, as has been said, are arranged in a single 
row on the surface of the periphery of the organ, this zone being classed 
by the older writers as the membrana eboris. Like the ameloblasts of 
the enamel organ, the odontoblasts do not make their appearance until 
the papilla has assumed certain proportions, this about corresponding 
to the size and form of the dentin of the future tooth. Immediately 
beneath the layer of odontoblasts appears a zone almost devoid of cells. 
This is followed by a district in which the cells are quite numerous, and 
finally when the central portion of the papilla is reached the cells are again 
few in number and widely scattered. For the most part the cells in the 
interior of the papilla are spindle-shaped or stellate, having rounded 
nuclei about which there is a small amount of protoplasm which pene- 
trates the intercellular substance by numerous hair-like processes. 

The Odontoblasts. — These are club-shaped or flask-shaped cells, 
each provided with a large nucleus which usually assumes the outline of 
the enlarged end of the cell which is directed toward the interior of the 
papilla. From the opposite end of the cell, or that directed toward the 
enamel organ, and in close proximity to its concave surface, one or more 
protoplasmic processes are given off. These persist and are finally encap- 
suled within the calcified dentin, forming the dentinal fibers. These 
cells are very closely associated, so much so, in fact, that their enlarged 
1 7 



258 ANATOMY. 

extremities are almost or quite in actual contact, more or less space ex- 
isting between the constricted portion of the cells as they pass toward 
the surface. 

Germs for Permanent Molars. — Reference has been made to the 
fact that the enamel organs for the deciduous teeth are given off from the 
tooth-band at a point somewhat distant from its free margin, so that the 
tooth-band is continued beyond the primitive enamel germ, this free 
margin of the band afterward generating the enamel organ for the succe- 
daneous tooth. As the twelve permanent molars are not succedaneous 
teeth, some other means must be provided for their development. 

Opinions of various writers upon this subject are somewhat conflict- 
ing. The theory is advanced by some that as the jaw increases in length 
the tooth-band and lamina primarily provided for the deciduous teeth 
are extended backward, first giving off a bud for the first permanent 
molar; at a somewhat later period, and with the increase in the growth of 
the jaw, an additional bud is generated for the second molar, the third molar 
being provided for in a like manner. Another theory, and one generally 
accepted as correct, is that the cords for the permanent molars spring 
individually and directly from the subepithelium. There may be found 
an exception to this in the case of the first permanent molar, which some- 
times appears to have its origin from the distal follicular wall of the second 
deciduous molar. Whatever theory be accepted in regard to the genesis 
of these permanent organs, the process of development after the appear- 
ance of the primary bulb or enamel germ is identical with that of the de- 
ciduous teeth. 

The Dental Follicle, or Tooth-sac. — During the early life of the 
tooth-germ, both the enamel organ and the dentin papilla are differen- 
tiated from the surrounding parts by dissimilarity of structure only, but 
as development proceeds, a more definite separation appears between the 
tooth-generating organs and the general tissues of the primitive jaw, 
this separating medium being known as the dental follicle. The term 
"follicle" is only one of a number applied to these parts, "dental saccu- 
lus," "tooth-sac," and other appellations being employed with equal 
significance. By some writers it is customary to apply the term " follicle " 
up to the period of complete closure, the term "sac" or "sacculus" 
being employed after that time. There appears, however, to be little 
foundation for such a distinction, the terms being synonymous. Again, 
the follicle is frequently referred to as meaning the sac and its contents, 
but this usage is a misapplication of the term. There appears to be no 
well-founded reason why the follicular wall or sac should not be referred 



DEVELOPMENT OF THE TEETH. 



259 



to as such, regardless of the formative organs within. As to the develop- 
ment of the tooth-follicle, it appears to be a generally accepted theory 
that at a very early period there is developed from the base of the papilla 
cells which, differentiating, form the walls of the follicle. By this growth 
of cells the periphery of the papilla is first surrounded, and this step is 
soon followed by an extension of the cellular structure in the direction 
of the surface epithelium, to the deep layer of which the cells become 
firmly attached, and in so doing inclose the enamel organ, which hangs like 
a hood over the extremities of the papilla. The tissue thus formed from 



Calcified Dentin 



Dentin Papilla, 
or Tooth-pulp 



Wall of Follicle 



Developing Bone 




Wall of Follicle 



Developing 
Bone 



Fig. 194. — Developing Tooth about the Fourth Fetal Month. Appearance of 

the Tooth-follicle. 



the base of the dentin germ is continuous with and similar in its origin to 
the pulp-substance. The primitive tooth-germ, during the formation of 
the follicular wall, is found swinging in a membranous pocket, being 
supported by the epithelial band, which, in turn, is attached to the oral 
epithelium; but as the walls increase and completely inclose the germs, 
which is accomplished about the fourth fetal month, the epithelial band 
is broken and the second or saccular stage of tooth-development is reached. 
The walls of the follicles are made up of two layers; the outer layer is 
dense and firm, and finally becomes the dental periosteum; the inner 
layer is thin, frail, and in the recent state somewhat transparent, and at 
an advanced period assists in the formation of the cementum. 



260 



ANATOMY. 



Having thus briefly described the primary stage of tooth-develop- 
ment, the careful study of which can only be pursued with the aid of the 
microscope, we will now pass to the secondary or saccular stage. By 
the introduction of a number of illustrations, prepared from original 
dissections by the author, this phase of the subject will be readily 
comprehended. 

Before continuing the subject of tooth-development, it will be 
eminently proper to briefly describe those parts directly concerned in the 
process. At a very early period of fetal life we find preparations are being 



Calcified Dentin 
Enamel 

Outer Enamel 

Epithelium 



Epithelium of 
Jaw 



Dental Ridge 



Germ for Perma- 
nent Incisor 




Dentin Papilla 



l'n;. 195. — Development of De< iduous Im isnr, from Human Fetus.- 

(Afler Cyst.) 



made for the development of the maxillary bones. That these are about 
the first bones to be called into functional activity accounts in a measure 
for their very early development. The osteoblastic activity in the inter- 
cellular substance destined to become the inferior maxilla begins about 
the middle of the second month of fetal life, while at a somewhat later 
period a similar action takes place in the region of the superior maxilla. A 
detailed description of the body of these bones having been given on another 
page, it will not be repeated here, but to that portion which gives lodg- 
ment to the tooth-germs, and which in a measure is controlled by their 
presence, some attention must be given, and for this purpose the mandi- 
ble will principally be used. 



DEVELOPMENT OF THE TEETH. 



26l 



Fig. 196 represents the lingual face of the lower jaw after removal 
from a three months' fetus. Attached to it is the remaining portion of 
Meckel's cartilage, which by this time is much wasted. It will be re- 
called that this cartilaginous band appears in the mandibular processes 
before the beginning of the second fetal month, being formed in two 
distinct halves, the free ends of which finally unite at the median line, 
forming a continuous support or framework, about which ossification 
takes place. At a corresponding period, and in a similar manner, two 
like processes are thrown out for the superior maxilke, but, unlike 
Meckel's cartilage, these do not unite at the median line, but stop short 
of this, the space thus resulting being provided for by two additional 

That Part of Cartilage not yet Ossified. 



Head of Malleus 
Cartilage of Incus 

Handle of Malleus 



, 


1 
) 



Ossified Mandible 
Fig. 196. — Developing Mandible, Three-month Fetus. 



processes, which shoot down from the region of the forehead and provide 
for the development of the intermaxillary bones. About the middle of 
the second month a center of ossification appears in the neighborhood 
of the future mental foramen, quickly followed by others at the symphy- 
sis and at the angle. These secondary centers soon unite with the 
primary one, and by the end of the second fetal month the osseous con- 
tour of the primitive jaw is established. While ossification takes place 
in the membrane surrounding Meckel's cartilage, the cartilage itself 
does not appear to be directly concerned in the process, and by the sixth 
or seventh fetal month the mandibular portion completely disappears, 
while that portion near the tympanum is ossified into the malleus. That 
portion of the bone which forms above Meckel's cartilage and the inferior 
dental nerve is that which finally gives support to the tooth-germs. This 
cartilage is not confined to the human species, but is the common herit- 
age of reptiles, rodents, birds, and fishes, in all of which it gives support 
to the developing lower jaw. 



262 



ANATOMY. 




* 



Figure 197 represents the evolution of the mandible from the middle 
of the third fetal month to the time of birth. It will be observed that 
during this interval there is a gradual increase in the size of the bone, 
but little alteration in its contour. By a constant and gradual osseous 

deposit about the distal extremity of 
the bone its length is increased to accom- 
modate the additional teeth as they make 
their appearance. While the external 
form of the bone shows but slight 
variation during this period, the internal 
structure, or that wherein the tooth- 
germs lie, is undergoing a complete 
transformation. 

Figure 198 is illustrative of these 
changes; beginning with a simple groove, 
or gutter, into which the tooth-follicles 
hang, the follicles exerting a controlling 
influence over its form. Next comes the 
appearance of septa between the anterior follicles, which at this period 
are somewhat irregularly placed in the arch, followed in a few weeks by 
a well-defined partition between the cuspids and molars, until finally, at 
birth, each follicle is inclosed in its individual crypt, with the single 
exception of the second molar, 
in which the distal septum, or 
that which is to separate it 
from the permanent first 
molar, has not yet made its 
appearance. As the tooth- 



FlG. 197. — Evolution of the Mandi- 
ble from the Third Fetal Month to 
Birth, Two-thirds Actual Size. 




follicles increase in size, by 
the development of the teeth 
within, they become more per- 
fectly inclosed in the bony 
vaults, the sides of the 
alveolar walls arching over 
and almost completely in- 
closing the developing teeth. 
Figure 199 shows the lower jaw of a seven-months-old child 
embodying the condition above referred to. No sooner have 
the crypts grown to this extent, than the resorptive action pro- 
duced by, or provided for, the advancing crowns speedily results in 



Fig. 198. — Evolution of the Mandible from Third 
Fetal Month to Third Month after Birth. 



DEVELOPMENT OF THE TEETH. 



263 



their downfall, to be again built up with the evolution of the permanent 
teeth. 

About the first visible sign of preparation for the development of the 
teeth, other than that made apparent by dissection, may be observed as 



Mucous Membrane and Periosteum Lifted Up 




Fig. 199. — Inferior Maxilla of Seven-months-old Child. 

early as the beginning of the third fetal month, when, upon opening the 
cavity of the mouth and looking upon the palate, a well-defined infold- 
ing of the epithelial eminence will be seen. In figure 200 this condition 
is shown by a dissection through the oral cavity of a four months' fetus, 
the outer fold being that of the cheeks and lips, while within are the hard 



Labiodental Space 



Labial Fold 




Primitive Dental 
Groove, so called 



Fig. 200. — Section Through the Mouth of Four-month Fetus. 

palate and primitive alveolar ridge. The mouth at this period has 
passed the rudimentary state, the transverse plates which contribute to 
the formation of the hard palate. having approached each other until the 
oral and nasal cavities, heretofore existing as a single buccal cavity, 
have become separate and distinct. The infolding of the oral epithelium, 



264 



ANATOMY. 



Maxilla 




as outlined on the summit of the primitive alveolar ridge — the primitive 
dental furrow so called — marks the position of the tooth-band, from 
which are given off the incipient tooth-bulbs. For the purpose of further 
investigation, a dissection of these parts was made and the maxillary 
bones removed, after which they were divested of their fibrous covering, 
including the periosteum. That portion which overlies the palatal 
processes was readily lifted in one sheet, while that upon the facial surface 

was separated at the median line and 
stripped independently of the other 
(Fig. 201). The removal of these 
tissues is readily accomplished until 
the margins of the partly formed 
alveoli are reached. Here the peri- 
osteum dips down into the various 
crypts, and serves as a lining mem- 
brane for them, and probably con- 
tributes fibers to the outer layer of 
the follicular walls. After advancing 
thus far, the detached tissues may 

FIG. 201. — Superior hetal Maxilla, 
showing manner of Dissection to E \ po se be grasped, and by careful manipula- 

Tooth-sacs. tion the tooth-follicles containing the 

formative organs removed from their respective vaults and turned over for 
examination. 

Figure 202 shows the result of such a dissection. On the left, the 
palatal plates and alveolar walls of the divested bones may be observed; 
on the right, is the fibrous covering, which has been turned completely 
over after removal from the bones, having firmly attached to it the 
ten tooth-follicles for the deciduous teeth. If this dissection be made 
without the precaution of lifting the periosteum, the follicles would not 
cling to the oral membrane with sufficient tenacity to permit of their 
ready removal. It may be of some interest to note that at this early 
period the position of the follicles containing the germs for the lateral 
incisors is that which the tooth is forced to occupy up to and frerpuently 
beyond the eruptive period, being crowded within the tooth-line by the 
central and cuspid follicles, in consequence of which the lateral crypts 
are thrown well into the palatal plates. 

Figure 203 represents the sacs broken down, exposing to view the 
dentin papillae, or those structures destined to become the tooth-pulps. 
The position which these occupy in the illustration is exactly the reverse 
from that which they assume when in position in the follicle, being thus 



DEVELOPMENT OF THE TEETH. 



265 



reversed that a better idea of their shape may be obtained. Prior to the 
twelfth or thirteenth week of fetal life this incipient bulb or papilla is 
without definite form; but by the latter period each papilla begins to 
assume the contour of the future tooth-crown, as faintly outlined in the 
illustration, those of the incisors presenting the angular form of the 

Tooth-follicles 




Maxilla 



Oral Mucous Membrane 



Fig. 202. — Dissection upon Superior Maxilla, Fourth Fetal Month, Exposing 

Tooth-follicles. 

future cutting-edge, the cuspids that of the single cone, while the coronal 
extremities of the molars are represented by outlines corresponding to 
the future cusps and marginal ridges. Besides the dentin papilla, there 
is contained within the follicular walls the organ which later on is pro- 
ductive of the enamel, but up to this time has been actively engaged in 



Dentin Papillae 



Maxilla 




Oral Mucous Membrane, Turned Completely Over 

Fig. 203. 

molding the tooth-form as outlined by the papilla. To return to the tooth- 
follicle, the dissection in this instance being upon the lower jaw of a 
four months' fetus. Figure 204 shows the mandible removed and the 
dissection carried to the point at which the follicles may be lifted from 
their bony encasements, this being accomplished by an incision along 



2 66 



ANATOMY. 



the base of the bone, followed by a stripping of the membrane first from 
the facial and then from the lingual side of the bone. When these two 
flaps reach the margin of the crypts, they are firmly grasped and the 
follicles removed from their sockets, as illustrated in figure 205. 

At the beginning of the saccular stage of development the form of 

Oral Mucous Membrane and Periosteum Dissected from Mandible 




Attachment of Follicle to Oral Membrane (Enlarged One-third) 
Fig. 204. — Manner of Dissection to Expose Tooth-sacs. 

the future tooth-crown is well outlined by the dentin papilla, which in 
figure 206 is brought into view by a dissection of the walls of the follicles 
shown in figure 205 without breaking the attachment existing between the 
two. As in the case of the follicle, much confusion of terms in regard to this 
structure exists, the "dentin bulb," the "pulp," "dentin germ," and the 



Tooth-follicles Oral Membrane 




Follicle or Perma- 
nent First Molar 



Fig. 205. — Tooth-follicles Removed from Mandible, Fourth Fetal Month. 



"papilla" being used ad libitum, without apparent regard for the struc- 
tural changes which are continuously affecting the organ. It will, there- 
fore, be proper to simplify this conglomeration of terms by a classification 
appropriate to the various stages of development. The term "dentin 
papilla" will best describe this part of the tooth-germ up to the time of 



DEVELOPMENT OF THE TEETH. 



267 



beginning of calcification, subsequent to which the term "pulp" should 
be employed. As previously stated, the enamel organ, until this period, 
has been principally devoting its energies to the molding of the tooth- 
form, and it is not until this model, as represented in the papilla, is com- 
plete that the process of calcification begins. About the fourth fetal 
month preparations for the calcification of the deciduous teeth are begun 
by the development of the odontoblastic cells for the dentin, which first 
made their appearance on the periphery of the dentin papilla, the summits 
of the various cusps in the molars and the future cutting-edges of the 
incisors being first affected. This phenomenon is soon followed by the 

Space Ocupied by Oral Mucous 
Enamel Organ Membrane 



Follicular Wall 




Dentin Papillze 
Prior to Beginning 
of Calcification 



Fig. 206. — Tooth-follicles shown in Figure 201 Opened. 



appearance of the ameloblastic cells for the enamel, which establish 
themselves in the internal epithelial layer of the enamel organ. 

Figure 207 is prepared from a dissection made in a manner similar 
to that shown in figure 203, but at a period about a month later, being from 
the superior maxilla of a five months' fetus. The dissection shows 
the extent of calcification at this period, which process also defines the 
position of the odontoblastic cells upon the extremity of the papillae. In 
the incisors (one of which was lost in the preparation of the specimen) 
the dentin may be seen capping the cutting-edges. The cuspids in this 
subject have not yet begun to calcify, although it is not unusual to find 
the cusp of this tooth receiving its lime-salts at this early period. In the 
molars the summits of the various cusps, as well as a portion of the various 
ridges descending therefrom, are undergoing the change produced by 
the impregnation of the lime-salts. It is quite probable that these delicate 
caps are at this time composed of dentin alone, the calcoglobulin which 



2 68 



ANATOMY. 



precedes the enamel calcification forming somewhat later. From this 
time forward the pulp undergoes a gradual transformation as to size and 
form, and there is likewise a change in its cellular construction on those 
parts adjacent to the calcific action. While the outline of the pulp is 



Calcified Caps Tooth-pulp 



Maxilla? 



Oral Mucous 

Membrane, 

Turned Over 




Fig. 207. 

gradually changing, its original form is permanently recorded upon the 
periphery of the dentin cap, which, when once formed, is immutable, all 
additions taking place from within. 

Figure 208 illustrates the result of a dissection upon the lower jaw 
of the same subject, disclosing practically the same conditions, with the 

Calcified Dentin Oral Mucous Membrane 



Space Occupied by 
Enamel Organ 




Dentin Papilla? 



Fig. 208.— Tooth-follicles Opened, Exposing Dentin Papilla; and Beginning of 

Calcification, Fifth Fetal Month. 

exception of the sac containing the developing cuspid, which was found 
with a slightly calcified cap of dentin. This slight variation between the 
development of the upper and lower teeth is one that is present in nearly 
every instance, the latter being somewhat in advance of the former. 



DEVELOPMENT OF THE TEETH. 



269 



In figure 209 the tooth-pulps, with their primitive cappings of dentin, 
have been removed from the follicles, and a better opportunity of study- 
ing the relations between the two parts is presented. By the conversion 
of the coronal extremities of the dentin papilla into odontoblasts, and 
their active calcification, some positive union between the two parts 
might be expected. On the contrary, the dentine caps are readily 



• + 



M* 



Fig. 209. 

removed, leaving the pulp beneath without the slightest rupture, so that 
we find calcification is not a secretory or excretory metamorphosis, but 
that the change takes place within the substance of the papilla itself, 
whereby it is altered from an organic to an inorganic substance. 

The next dissection was one upon the mandible of a six months' 
fetus. Figure 210 shows the tooth-follicles removed from the partially 
formed bony crypts in which they have been incased. x\t an early 



Tooth-sacs of 
Deciduous Teeth 




Lingual Surface of Mandible 
FlG. 210. 



period of fetal life, and at a time prior to the completion of the tooth- 
follicles, there is deposited beneath the tooth-germs a thin layer of bone, 
which at once begins to assume the form of the partially developed fol- 
licular walls. As the growth of the follicle proceeds, there is a correspond- 
ing increase in the osseous deposit, the alveolar walls extending about 
and accommodating themselves to the membranous sacs. Thus we find 



270 



ANATOMY. 



in this portion of the maxillary bones a feature peculiar to itself — that 
of a continuous transformation from its earliest inception to the adult 
period, first developing about the temporary tooth-sacs and completely 
incasing them, which is speedily followed by complete resorption of the 
walls, again followed by a rebuilding during the evolution of the perma- 
nent teeth, and again swept away with the loss of these organs. Figure 211 

Follicular Wall Calcified_Caps Oral Mucous Membrane 




Dentin Papilla 
First Perma- 
nent Molar 



Tooth-pulps 

Fig. 211. 

illustrates the opposite side of the same jaw, with its outer or facial plate 
removed, together with the intervening septa. The follicles are opened, 
and the extent of calcification at this period (six months) made apparent. 
The pulps and calcified caps are approximately as found when dissected, 
save a slight settling of all the parts. The incisors have calcified to about 
one-third their full coronal length; the unicusped .contour of the cuspid 
has been established, as shown by the deposit of the lime-salts upon its 




Fig. 212. 



summit, and the upbuilding of the mesial and distal cutting-edges. The 
first molar has about completed its occlusal surface, and, while the cusps 
of the second molar are nearing completion, there is a lack of union in the 
central and distal fossae. Immediately posterior to the second deciduous 
molar, the sac containing the formative organs for the first permanent 
molar is shown opened, exposing to view the dental papilla, which at 



DEVELOPMENT OF THE TEETH. 



271 



this early period has assumed the form of the future tooth-crown, and 
calcification is about to begin. Figure 212 illustrates the extent of calcifi- 
cation in the deciduous teeth at the sixth fetal month. As the growth of 
the teeth proceeds, it will be observed that the angularity which originally 
accompanied the calcifying caps of the molars is gradually disappearing, 
as is also the tritubercular form of the incisors and cuspids, this 
change being brought about by the deposition of enamel to the parts. 
Figures 213 and 214 show the calcified caps removed from the 




Fig. 213. 

pulps, and so arranged that both their external and internal anatomy 
may be studied. Prior to this time there has been but little alteration in 
the form of the dentin pulp, only a gradual decrease in its size being 
noted; but now we find it being divested of many of its angles, particularly 
those which originally served as a basal form for the coronal extremities 
of the future tooth. With the disappearance of these the concavity within 
the cap is slowly assuming the form of the future pulp-chamber. 

That a better understanding of the saccular stage of tooth-develop- 




Fig 214. 



ment might be had, a transverse section was made through the molar folli- 
cles, as shown in figure 215. In this the attachment of the follicular walls 
to the deep epithelial layer is visible, while within the walls is the enamel 
organ, the calcified dentin, and the tooth-pulp. As previously stated, 
the enamel organ is seen suspended above and forming a hood-like invest- 
ment to the calcifying structure. This organ not only overhangs the occlu- 
sal surface of the tooth-crown, but completely envelops the sides of the 
calcified cap and dentin pulp. Previous to the beginning of calcification 



272 



ANATOMY. 



the enamel organ is in close proximity to the dentin papilla, the original 
form of the latter being represented by the calcified dentin. 

We have now arrived at that period of fetal existence when it is possi- 
ble to study the macroscopic development of the permanent first molar. 
Figure 216 represents a section of the lower jaw of a six months' fetus, 

Enamel Organ Oral Mucous Membrane or Gum 




Follicular Wall 



Tooth-pulp Floor of Tooth-follicle 
FlG. 215. — Dissection Showing Pulp, Calcified Cap, and Enamel Organ. 

and displays not only the sacs of the deciduous teeth, but also that of 
the permanent first molar. In most respects the evolution of this 
tooth is similar to that of the temporary organs, having its origin from 
the deep epithelial layer, either directly or by continuation of the tooth- 
band backward. Preparations for its growth are begun as early as 
the third fetal month, at which time the enamel organ is given off, and 
oral Mucous Membrane thereafter the developmental process is 

identical with that of the deciduous teeth. 
There is one structure, however, 
intimately connected with the develop- 
ment of the permanent teeth not found 
in connection with the deciduous organs 
— the gubemaculum, or leading cord. 
Figure 217 represents another section of 
a six months' fetal mandible, with the 
dentin papilla for the permanent first 
molar turned out from the follicle after being rolled from its bony 
incasement. Attached to the apex of the tooth-sac (which has been 
turned back), and leading from it to the epithelium of the jaw, is the 
gubemaculum. This fibrous structure was at one time thought to be 
directly concerned in the development of the tooth. Although this is 
denied at present, little is said in regard to its function, but it undoubtedly 




Fig. 216. 



DEVELOPMENT OF THE TEETH. 



2 73 



serves the purpose of directing the tooth to that position which it should 
occupy in the jaw, and where the least resistance to its eruption is 
formed by the foramen which the cord has established. Each of the 
permanent teeth is provided with a similar membranous cord, an illus- 
tration and more complete description of which will follow later on. 

We have now arrived at a period when the subject under consideration 

Oral Mucous Membrane Gubernaculum 



2Ef^ 



Dentin Papilla 
FlG. 217. 

naturally becomes of deeper interest. I refer to that time when the being 
changes from a complex dependent condition to one of self-providing inde- 
pendence. Previous to the time of birth the teeth appear to be but little dis- 
turbed by certain morbid conditions which might be present in the parent, 
and from their earliest inception up to this period their development pro- 
ceeds with but little interruption and with much regularity. Figure 218 



I 1 4 
ft i * 



Fig. 218. — Deciduous Teeth at Birth. (Reflected picture.) 

illustrates the condition of the deciduous teeth at birth; the central incisors 
are calcified externally to the cervical line, the lateral incisors to a point cor- 
responding to the summit of the palatocervical ridge; the cuspids have ad- 
vanced somewhat beyond the angles of the crown, while the molars have 
their crowns calcified to about one-half their completed length. With all 
this progress as represented by the external contour of the tooth-crowns, 
18 



274 



ANATOMY. 



the internal form appears to be somewhat slow in assuming the shape of 
the future pulp-chamber. From the beginning of the saccular stage of 
development up to the time of birth there is but little increase in the 
diameter of the tooth-sac, but there occurs a gradual increase in its length. 
Figure 219 shows the mandible from a child one week old, with the greater 
part of the external or facial surface of the bone removed, exposing 

Oral Mucous Membrane 




Fig. 219. 

not only the sacs containing the developing deciduous teeth, but also 
that of the permanent first molar. The relation of the sacs to the inferior 
dental canal is apparent, as well as the firm attachment of the follicular 
walls to the oral membrane. In figure 220 the tooth-sacs have been dis- 
sected and the pulps removed from the calcified caps, presenting an 
additional illustration of the amount of dentin deposit at this age. To 

Calcified Caps 



Dentin Papilla of 
Permanent First 
Molar 




Pulps 



Fig. 220. 



further illustrate the size and form of the pulp as compared with the 
calcified cap at birth, a transverse incision was made through the left supe- 
rior maxilla, at a point corresponding to the base of the pulp, as shown 
in figure 221. The calcified parts remain in position resting against the 
remaining portion of the enamel organ, while the pulps are dislodged and 
may be observed resting upon the incised surface. In this illustration 



DEVELOPMENT OF THE TEETH. 



275 



the dentin papilla for the first permanent molar is also seen, being sup- 
ported by the walls of the follicle, which in turn are attached to the oral 



Calcified Caps, Resting against 
Enamel Organ 




Tooth-pulps Rolled Out 



Section of Superior Maxilla 



Pulp of Permanent First Molar 

Fig. 221. 



epithelium by the gubernaculum. It may also be noted that those parts 
of the pulp corresponding to the cusps and marginal ridges show a decided 
convergence of the surface toward the center. 



Dentin Papilla for 
Permanent First 
Molar 

Papilla 

Calcified Cap 



Walls of Tooth- 
sacs 






Maxilla 



Oral Mucous 
Membrane 



Fig. 222. 



Reference has been made to the formation of the follicular walls by a 
differentiation of cells, which at an early period are given off from the 



276 



ANATOMY. 



base of the papillae, and to the continuity of the two structures. Figure 
222 was prepared for the purpose of showing these intimate relations. 
The sides of the sacs were opened and turned back; the calcified tooth- 
caps with pulps in position were grasped and given several revolutions, 
thus twisting the remaining portion of the walls, the floor of which is 
seen as a continuous structure given off from the pulp and connecting 
it with the epithelium of the jaws. 

PREPARATIONS FOR THE DEVELOPMENT OF THE 
PERMANENT TEETH. 

A little before the time of birth sufficient advance has been made in 
the development of the permanent teeth to permit a study of their rela- 
tions with the temporary organs. The early preparations for the growth 
of these teeth was for a long time a subject of much controversy, some 
writers advancing the theory that the buds for the permanent teeth were 
produced or given off from the sacs of the temporary teeth ; others contend- 







#•-, 




Tooth-sac of Advancing 




| | 

MB ^^^k^ 


to 


Deciduous Tooth 




Gum 


-v 


*"^"^ 





Gubernaculum Tooth-sac o£ Receding Permanent Tooth 
Fig. 223. — Section through Superior Maxillae, Sixth Month after Birth. 

ing that the cords were derived from the remnants of the primitive cords 
immediately after their rupture. The theory now generally accepted 
is that the cord is given off from the primitive cord at a point in close 
proximity to its attachment to the deciduous enamel organ. This can 
only apply to those teeth which are succedaneous, and, therefore, does 
not include the permanent molars, as heretofore stated. Whatever 
theory be accepted in regard to the genesis of the permanent teeth, there 
can be no mistake in regard to that part of the process which we are 
permitted to ocularly investigate. I shall, therefore, proceed to describe 



DEVELOPMENT OF THE PERMANENT TEETH. 



2 77 



the position and contents of the permanent tooth-sacs at birth. 
Figure 223 presents the result of a vertical dissection through the su- 
perior maxillary bones, the inner surface of the right maxilla being 
exposed to view. Many of the frail processes, particularly those en- 
tering into the construction of the nasal cavities, were lost during 
the preparation of the specimen, the whole purpose of the dissection 
being to show the sac of the permanent incisor and its relation- 
ship to its predecessor. By this time calcification in the deciduous 
incisor has so far advanced that the contour of the tooth-crown may be 
plainly outlined through the walls of the sac. Resting against the palatal 



Foramina for Gubernacula 



,.._,„ 






jL^ 


m *' 


* |L 


pi 


•*^Bl_ 


>~ 


*•» ^| ^Pv| 


-. ■& 




H 


* 



Fig.. 224. 

concavity of the crown of the temporary incisor is the sac containing the 
formative organs for its permanent successor. This permanent tooth- 
sac does not long remain in such close proximity to the deciduous tooth- 
crown, for as the latter advances toward the surface of the gum the former 
recedes and is soon inclosed in a separate crypt, which, were it not for 
the gubernacular foramen, would completely inclose it. Figure 224 
represents a section of the left superior maxilla, introduced at this point 
for the purpose of showing the position of the foramina for the gubernacula. 
These may be observed immediately posterior to the incisor and 
cuspid teeth. At birth these foramina do not exist as such, the partially 
formed vaults containing the sacs for the permanent teeth appear- 
ing as an extension of the temporary crypts in a palatal direction; but as the 



278 



ANATOMY. 



temporary teeth advance and the permanent teeth recede, the roof of the 
crypt is completed, and the foramen established by the presence of the 
gubernaculum. This extension of the temporary crypts ts more clearly 
demonstrated in figure 225, which represents one side of the lower jaw at 
birth with the partially calcified deciduous teeth in position in the bone. 

Tooth-sacs for Permanent Incisors Gum 




Tooth-sac for Perma- 
nent First Molar 



Deciduous Teeth in Bony Crypts 

Fig. 225. 

Suspended above this is the gum, which has been dissected from the 
bone, having attached to its under surface the sacs for the permanent 
teeth. Those for the incisors are particularly well defined and their 
place of lodgment in the bone readily noted. In the case of the cuspid, 
both the deciduous tooth and the permanent tooth-sacs are in position 
in the crypt. The cords which support the incisors are somewhat length- 

Oral Mucous Membrane 



Follicular Wall, Turned 
Wrong Side Out 




Papilla for Permanent First 
Molar 



Dentin Papilla? for Permanent Teeth, Upside Down 

Fig. 226. 



ened from the weight of the sacs, the gubernacula not assuming this 
thread-like form until the permanent sacs have further receded. 

Figure 226 shows the result of a dissection upon these permanent 
tooth-follicles. The dentin papillae of the various teeth are seen in a 
reversed position, with the follicular walls attached to their bases. At 
this period the papillae for the permanent incisors may be compared to the 



DEVELOPMENT OF THE PERMANENT TEETH. 



2 79 



tail of a fish, being perfectly transparent over its free extremity, which 
feature is gradually lost as its thickened base is approached. The fish- 
tail appearance is further represented by the division of the free extremity 
into three distinct parts, each of which provides a separate point of calcifi- 



Tooth-sacs of Permanent Teeth 



Tooth-sacs of 
Deciduous Teeth 




Lingual Surface of Mandible 

Fig. 227. 



Tooth-sacs of Deciduous Teeth 



Tooth-sac of Permanent Tooth 



Periosteum and Mucous Mem- 
brane from Hard Palate 




Gubernaculum 



Tooth-sacf or Permanent First 
Molar 



Fig. 228. — Same as Figure 227, Except on Upper Jaw. 

cation. This cuspid papilla is missing, and the bicuspids have advanced 
little beyond the form of the primitive bulb. The first molar is shown 
with the full diameter of the crown represented by the pulpal mass, and 
the tips of the cusps are already beginning to take on the calcific 
action. 



28o 



ANATOMY. 



A further illustration of the progress of the development of the 
permanent teeth and their relation to the deciduous organs may be seen 
in figure 227, the dissection in this instance being upon the lower jaw of 
a one-month-old child. The membrane has been lifted from the bone 
with the tooth-sacs attached to it. Immediately posterior to the sacs 
containing the crowns of the temporary incisors and cuspids are those 
,for their corresponding successors, the papillae of which have already 
assumed the tubercular outline of the future cutting-edge. While the 
permanent tooth-sacs are distinctively independent pouches, there ap- 
pears, nevertheless, to be a well-established fibrous connection existing 



Tooth-sacs of 
Permanent Teeth 




Tooth-sacs of 
Deciduous Teeth 



Periosteum of Hard Palate 

FlG. 229.— Tooth-follicles fur Deciduous and Permanent Teeth, Three Months 

after Birth. 



between the outer layer of the two follicular walls. This fibrous union 
is gradually broken as the permanent sacs recede and become incased 
in their own vaults. 

While the permanent tooth-sacs are generally referred to as "reced- 
ing," it is a question if this term is fully justified. While the follicles 
do not remain in close relation with their predecessors, the change in the 
relative position of the two is principally brought about by the advance 
in the deciduous sacs, this forward movement being accompanied by a 
marked growth of the bone in the direction of the future alveolar ridge, 
thus leaving the permanent tooth-sacs well buried in the substance of 
the jaw. 

Figure 229 represents the result of a dissection upon the superior 
maxillae of a three-months-old child. The mucous membrane covering 
the hard palate, together with the periosteum, has been dissected from 
the bones and turned over for examination. The tooth-follicles for all 



DEVELOPMENT OF THE PERMANENT TEETH. 281 

the deciduous teeth, as well as those of the succedaneous permanent 
organs, may be observed firmly attached to the fibrous tissue. The 
permanent incisor sacs at this age are almost equal in size to those of 
their predecessors, and the dentin papillae within possess a mesio-distal 
diameter almost equal to those of the calcified temporary caps. The 
sacs containing the germs for the permanent cuspids and biscuspids are 
somewhat diminutive, but the enamel organs within are already molding 
the contour of the future tooth-crowns upon the dentin papilla-. 

By the beginning of the second month after birth calcification in the 
crowns of all the deciduous teeth is about complete, and preparation for 
growth of the roots is under way. While at this period the tooth-crowns 
may be said to be almost completely calcified, this does not apply to the 






Fig. 230. 

interior of the crowns, the deposit of dentin internally being a continuous 
process, resulting in a gradual reduction in the capacity of the pulp- 
cavity. It is also quite probable that the enamel organ is somewhat active 
up to the eruptive period, and, if this be true, the enamel covering of the 
crown is not complete until this time. Whatever be the condition in the 
crowns, the time for the formation of the roots has arrived, and it is princi- 
pally through the activity of the tooth-pulp that they are generated. 
We have seen that the contour of the tooth-crown was first molded upon 
the dentin papilla; so it is with the tooth-root: by a gradual elongation of 
the sac, accommodations are afforded the tooth-pulp for a corresponding 
growth. As the pulp lengthens out toward the future apex of the root, 
it is molded to the root-form, and calcification takes place by the genera- 
tion of odontoblastic cells upon the periphery of this organic root-form. 
While the process of root-formation in the single-rooted tooth maybe 
readily comprehended, the bifurcation or trifurcation of the molar roots 
presents a complication which calls for special reference. Figure 230 
will assist in explaining this phenomenon. In the illustration three 
deciduous molar crowns are shown, two of which are incased in their 



282 ANATOMY. 

tooth-sacs, the third being stripped of its membrane. The view is directly 
upon the base of the tooth-sacs, immediately beneath which is the 
base of the pulp. Up to this period the odontoblastic cells have been 
generating about the occlusal surface and lateral walls of the crown only, 
but now an accumulation of these cells is to be found upon the base of the 
pulp, lining up in the position of the future root-walls. This structural 
change is faintly outlined in the illustration. By this inward extension 
of the odontoblastic cells from various points about the margins of the 
pulp, and their union near the center of the mass, provision is made for 
the calcification of the various roots, which process is considered separately 
by an extension and molding of the pulp into two or more divisions. 

Calcification of the Cementum. — While the dentin of the root is 
derived from the tooth-pulp, the external covering of the root (the cemen- 
tum) is generated from another source. In every respect cementum is 
closely allied to bone, and we find its development provided for in a 
similar manner. As stated in another part of this chapter, the tooth- 
sac is made up of an outer and an inner layer, both of which are rich in 
blood-vessels. These membranous walls continue to invest the roots 
of the teeth during their upbuilding. The outer layer of the sac remains as 
a permanent structure placed between the root and the alveolar walls, 
forming the alveolodental membrane, while upon the surface of the inner 
layer osteoblasts (cementoblasts) are developed, which are speedily 
converted into bone or cementum. In this process the tooth-root may 
be compared to one of the long bones of the body, and the development 
of the cementum considered under the head of Subperiosteal Ossification. 
The only variation to be observed between this and subperiosteal develop- 
ment of bone is in the presence of a single Haversian canal (as the pulp- 
cavity may be considered), and even this difference is sometimes over- 
thrown by small canals running at right angles to the pulp. These small 
canals are generally found near the apex of the root, at which point the 
cementum is the thickest. Like the enamel cap of the tooth-crown, the 
cementum is deposited upon the surface of the dentin of the root, thus 
increasing its diameter. 

Eruption of the Teeth. — -Up to this time no reference has been 
made to that process by which the teeth burst forth from their bony incase- 
ments, and, penetrating the mucous membrane, made their appearance 
in the mouth. Attention has been called to the growth of the bone 
about the tooth-follicles — first forming beneath them as an open gutter, 
next surrounding their lateral walls and inclosing each follicle in a sepa- 
rate compartment, and finally each tooth becoming more completely envel- 



DEVELOPMENT OF THE PERMANENT TEETH. 283 

oped by an arching-over of the mouth of the bony vault. This condition 
in the maxillary bones is reached between the seventh and eighth month 
after birth, and, almost simultaneously with the completed incasement 
of the teeth by the bone, active resorption begins, that portion of the 
bone which was last in forming being gradually removed. The cause 
of the resorption of the bone may readily be attributed to the advance- 
ment of the tooth, but the forces which are responsible for this latter 
phenomenon do not appear to be clearly understood. In a general way, 
the advancement of the crown may be said to result from the elonga- 
tion of the root by the addition of dentin to its free extremity. 
But, when it is taken into consideration that the cuspid teeth, both decid- 
uous and permanent, have their roots fully or nearly calcified before 
they begin to advance toward the surface, an exception to the generally 
accepted theory is established. The eruptive process takes place first 
in the anterior teeth,* and the bone overlying the labial surface is first 
removed. 

This loss of the bony structure is continued until fully one-half 
of the labial surface is uncovered, and, as the crowns continue to advance 
toward the surface, they assume a more prominent position in the arch, 
and thus their cutting-edges become bared. The palatal or lingual face 
of the crypt serves a double purpose, forming not only a covering to the 
deciduous tooth, but also serving the permanent tooth-sac in the same 
capacity. This part of the crypt remains unabsorbed, the tooth-crown 
glides by its margin, and, after penetrating the mucous membrane, makes 
its appearance in the mouth. Closely following the resorptive process 
comes a rebuilding of the parts, until, finally, when the tooth is fully 
erupted, it is firmly supported by the new bone filling in about the base of 
the root. Accompanying the eruption of the anterior teeth and their 
establishment in the arch is an increase in the depth of this portion of the 
jaw, and, as the molar teeth advance and assume their position, there is 
a corresponding increase in the depth of the jaw in this locality. At the 
beginning of the eruptive period the roots of the deciduous teeth are but 
partially calcified, but as the crowns advance the calcific action at the 
extremity of the roots is continued, and, in the majority of instances, by 
the time the crowns are fully erupted the roots are completely formed. 
During the period of eruption the transitory nature of the alveolar por- 
tion of the jaw-bone is made manifest, accommodating itself to the 
growth of the teeth as well as to their change of position. The free 
margins of the alveolar walls are taking on new structure, which ad- 

*See Description of the Teeth in Detail. 



284 



ANATOMY. 



varices with, and becomes adapted to, the base of the tooth-root. Coin- 
cident with this the deeper portion of the alveolar process is formed 
by a rapid filling-in about" the root as the tooth travels onward to assume 
its final position in the jaw. The eruption of the teeth is usually by 
pairs, with a slight intermission between each class. The central in 
cisors first make their appearance, followed by the laterals, after which 
the first molars are erupted. The cuspids usually follow the first 
molars, and, finally, the second molars take their place in the arch. 
While this brief description of the eruption of the teeth refers to the de- 
ciduous organs only, the process in the permanent teeth is almost 
identical with this. Further reference to the eruption of the perma- 
nent teeth will be made in connection with the degeneracy of the 
temporary set. 




Fig. 231. — Hard Palate from a Nine-months-old Child, Actual Size 



To return to the subject of tooth-development, attention is called to 
figure 231, prepared from a dissection upon a nine-months-old child, 
the illustration representing the hard palate, or roof of the mouth, at this 
period. The four incisor teeth have made their appearance, the labial 
surfaces of the crowns being fully exposed, while those facing the palate 
are but slightly uncovered. The approach of the remaining deciduous 
teeth is plainly indicated by the fullness of the alveolar borders, and the 
margins of the crypts are now being removed by resorption. 

If the mucous membrane should be removed, the crowns of the 
advancing teeth would be brought to view after the removal of the walls 
of the tooth-sacs, while the approaching cuspids and second molars yet 
remain partly covered by an arching over of the walls of the crypts; 



DEVELOPMENT OF THE PERMANENT TEETH. 



285 



the resorptive process has also begun in these parts. It would also be 
observed that, while the walls of the crypts are molded to the outlines 
of the tooth-crowns, there exists a well-defined interspace between the 
two. During the growth of the tooth this interspace is filled by the walls 
of the tooth-sacs, and even after the teeth have passed the saccular stage 
of development, and assumed their positions in the mouth, there yet 
remains between the roots and the alveolar walls a slight space which 
is occupied by the alveolodental membrane. 

The next dissection is one upon the superior maxilla? of a two-year- 
old child (Fig. 232), representing the roof of the mouth of this subject. 




Fig. 232. — Hard Palate from a Two-year-old Child, Actual Size. 

In this specimen it will be noticed that all of the deciduous teeth are 
erupted with the exception of the second molars. The lingual surfaces 
of the incisors are fully uncovered, while in the cuspids the labial surfaces 
are much more exposed than the lingual. In figure 233 the mucous mem- 
brane and sufficient of the bone have been removed to expose the tooth- 
sacs of the developing permanent teeth. The dissection furnishes no 
additional information over that obtained from figure 232, excepting 
that the primitive follicles for the permanent second molars make their 
appearance at this time. 

At this early period the jaw has not lengthened sufficiently to permit 
of these follicles occupying their future position; consequently they are 
found generating immediately over the tooth-sacs of the first permanent 
molars. As the first molars advance and the jaw lengthens backward, 
these follicles will be carried to the distal by the extension of the mucous 



286 



ANATOMY. 



membrane, to which they are firmly adherent. If these follicles were to 
be dissected at this time, the papillae would be without definite form, 
showing that the early function of the enamel organ has not yet begun. 
The tooth-sacs containing the permanent lateral incisors are found 
immediately beneath the palatal plates, and frequently during their 
earlier life they are not even protected by the bone, being in immediate 
contact with the mucous membrane. On account of the imperfect 
protection frequently afforded these sacs, the germs are sometimes 
injured and the teeth fail to make their appearance. 




Sac for Perma- 
nent Second 
Molar 



Fig. 233. — Roof of the Mouth of a Two-year-old Child. 

In figure 234 the walls of the sacs shown in figure 233 have been opened, 
and the relations existing between the first and second dentition at the end 
of the second year become apparent. The crowns of the permanent in- 
cisors are deeply set in the substance of the jaw, while the partially calci- 
fied crowns of the permanent laterals are in close proximity to the palatal 
surface. The partially formed crowns of the permanent cuspids are still 
more deeply seated in the substance of the jaw than those of the central 
incisors, and are not visible in the illustration. In this connection it will 
be well to again refer to the gubernaculum, and to its function — that of 
directing the tooth to its proper position in the arch. By reference to 
the illustration the crowns of the permanent teeth will be observed head- 
ing in various directions, and, while in this instance there appears to be a 
general tendency for them to advance and assume their proper positions 
in the arch, in many cases they will be found directed at right angles to 
the point at which they should emerge from the bone. The gubernaculum, 



DEVELOPMENT OF THE PERMANENT TEETH. 



287 



which appears to be nothing other than an elongation of the follicular 
walls, not only directs the tooth by the tension of its fibers, but the fora- 
men which its presence creates stimulates the resorptive action over the 




Fig. 234. — Development of the Teeth about the Second Year. 

tract to be traveled by the tooth. Figure 235 was prepared for the pur- 
pose of better showing the gubernacula, and the manner of connecting 
the tooth-sacs with the oral mucous membrane. This condition is 



Guber- 
naculum 




Tooth-sac for First Bicuspid 
Fig. 235. — Dissection of Lingual Face of Lower Jaw, Child Nine Months Old. 

well shown in the anterior teeth, the tooth-crowns having receded well 
toward the body of the jaw. The follicle for the first bicuspid may be 
observed attached to the lingual face of the deciduous molar sac, and 



288 



ANATOMY. 



the leading cord is not yet an adjunct to the developmental process, but 
this structure will make its appearance as the follicle recedes from the 
surface. 

In figure 236 the deciduous molars have been removed from the jaw 

and the relations existing between 
these teeth and the developing 
bicuspids is shown. The tooth- 
follicles for the succedaneous teeth 
are found immediately beneath the 
gingival margin, and apparently 
attached to the deep layer of the 
mucous membrane. This relation- 
ship between the permanent and 
temporary organs is present about 
the eruptive period, but as the 
deciduous tooth advances and the permanent tooth-sac recedes, the 
two organs become more widely separated, and the permanent follicle 
is connected to the surface only by the elongated follicular fibers which 
form the gubernaculum. 

Decalcification of the Deciduous Teeth. — By the close of the 
second year the twenty deciduous teeth have taken their place in the 




Fig. 236. — Deciduous Molars with 
Tooth-sacs for Bi< uspids Attached to the 
Gingiva] Tissui . 



Deciduous Molars Deciduous Incisors 




Permanent 
Incisors 



Permanent 
Cuspid 



Fig. 237. — Same as Figure 235, with Tooth-sacs Opened Showing Developing 

Teeth in the Jaw . 



dental arch, their roots have become fully calcified, and the apical fora- 
mina established; it is only for a short period, however, that they remain 
thus perfect, the process of decalcification beginning about the fourth 
year. This resorptive action begins at the apical extremities of the roots 



DEVELOPMENT OF THE PERMANENT TEETH. 



289 



and gradually progresses in the direction of the crowns. Commencing 
about the fourth year with the central incisor, decalcification takes place 
in the teeth in the order of their eruption, the lateral incisor following the 
central, the first molar following the lateral, etc. By reference to figure 238 
an approximate idea of the progress of decalcification may be obtained, 
and it will be observed that about three years elapse from the beginning 
of this rather obscure process to its completion, and the final casting-off 




Fig. 238. — Development of the Teeth about the Sixth Year. 

or shedding of the tooth-crowns. In reference to the causation of this 
dissolution of the deciduous teeth, but little appears to be known. It 
has been said to result from the presence and pressure of the advancing 
permanent teeth, but there is no question but that it. occurs absolutely 
independent of these organs, decalcification frequently taking place when 
from some obscure reason, one or more of the successional teeth are 
absent. During the entire period of root decalcification, the pulp of the 
19 



2 go 



ANATOMY. 



tooth, which is also involved in the destruction, retains its vitality, but 
with the loss of vitality in the pulp resorption of the root ceases; so that 
the gradual removal of the root-substance must be considered as a purely 
physiologic action. 

Figure 238 shows a dissection upon the jaws of a six-year-old child, 
by a careful study of which, a fair knowledge of the extent of resorption 
in the deciduous teeth at this period may be obtained. 

Advance of the Permanent Teeth. — By referring to figure 238, the 
relations existing between the deciduous and the permanent teeth at 










FlG. 239. — The Completed Dentition. 



about the sixth year may be noted. While the crowns of the deciduous 
teeth remain in position, a part of the space formerly occupied by their 
roots is taken up by the advancing crowns of the permanent teeth, the 
latter being calcified but little beyond their cervical lines. Between the 
seventh and eighth years the crowns of the deciduous incisors are cast 
off, and gradually the crowns of the permanent incisors force their way 
through the gum, the arch by this time having sufficiently increased in 
size to accommodate the additional width possessed by them. Previous 
to this time, or about the sixth year, by a backward extension of the jaws, 
the permanent first molars have erupted, assuming a position in the arch 
immediately posterior to the deciduous second molars. Between the 



DEVELOPMENT OF THE PERMANENT TEETH. 29 1 

tenth and eleventh years the crowns of the deciduous molars are lost, and 
the bicuspids advance to take their places. Usually by the twelfth year 
there has been sufficient increase in the length of the jaws to permit of 
an additional tooth, and the permanent second molar gradually takes its 
position immediately posterior to the first. Between the twelfth and 
thirteenth year the deciduous cuspids are lost by decalcification of 
their roots, and they are succeeded by the permanent cuspids. We 
therefore find, by the fifteenth year, fourteen fully developed teeth occupy- 
ing the dental arch of each jaw, the full number, thirty-two, or sixteen 
in each jaw, not being present until the eruption of the third molar, 
which, like the other teeth of this class, is compelled to await accommoda- 
tions by a further increase in the length of the maxillary bones. This 
tooth usually takes its place between the eighteenth and twenty-first 
years, and thus completes the dentition (Fig. 239). 



PART II.-HISTOLOGY AND HISTOGENESIS. 



CHAPTER I. 

General Cytology; General Embryology; and Histogenesis. 

The foregoing pages have been devoted to the description of the face 
and oral cavity, so far as they can be studied with the naked eye, either 
by close inspection, superficially as it were, or after their component parts 
have been made manifest by dissection. That part constitutes what is 
commonly known as gross or macroscopic anatomy. 

Similar to all other regions of the body, however, the functions of 
the individual parts composing the face and the oral cavity, in health, 
and the pathological processes which take place in them when they are 
diseased, are dependent on the normal or pathological condition of the 
minute structures or tissues, of which these parts are composed. These 
minute structures cannot be seen with the naked eye, they must be exam- 
ined with the aid of a microscope, and this part of study is therefore 
known as microscopical anatomy or histology — the science of tissues. 

The minute structures of the face and oral cavity, generally speaking, 
in no way differ from the structures of other parts of the body, but here, 
more than in any other region of the body, the anatomy as well as the 
histology of the parts cannot be well understood, unless their gradual for- 
mation or development — embryology — is studied. To enable the student 
of dentistry to understand more intelligently the details of the parts in 
which he is most interested, it is therefore essential to have a more or less 
thorough knowledge of the broad principles underlying, not only the 
mature structural arrangements of the human body, but also the develop- 
ment of the organs — organogenesis — as well as the development of the 
tissues of which these organs are composed — histogenesis. 

In submitting parts of different organs of the body to a more close 
examination with the microscope, we find revealed a great variety of 
more or less complex textural arrangements, each characteristic of the 
part examined, and adapted to the function which the given part has to 
fulfill. Some tissues we find consisting of either very thin or more or 

2 93 



294 HISTOLOGY. 

less coarse fibers running parallel to each other in a compact fashion and 
forming firm or elastic bands; others we find showing the same kind 
of fibers loosely arranged and interlacing with each other, thus forming 
more or less typical networks. In some instances we find similarly 
formed elements, so-called cells, joined together in a mosaic-like fashion 
and forming either coverings of surfaces or various tubes or sacs. The 
majority of organs consist of fibers and cells, modified and combined 
together in either a simple or more or less complex manner. We also 
observe that the substances filling the spaces between the fibers and 
cells, the so-called ground substances, vary in their consistency; it maybe 
a fluid or a semifluid, it may be of a more or less firm nature, or it may 
be very hard. Notwithstanding this great variety, however, all the 
tissues of which the animal body is composed can be grouped into five 
distinct classes, each one of which has its very well defined character- 
istics of structure, which makes it specially adapted to the various func- 
tions which it has to fulfill in the animal economy. These classes com- 
prise the elementary tissues, which by their various modifications and 
combinations form all the organs of the body. They are the following: 
i. Epithelial tissue. 2. Connective tissue. 3. Muscular tissue. 4. Nerv- 
ous tissue. 5. Blood and lymph. 

Before we attempt to describe the individual elementary tissues, 
it is necessary to become familiar with some features which are common 
to all of them. This can be best accomplished by giving a brief history 
of the beginning and gradual development of our knowledge on the subject. 

The principal features in the minute structure of animals are the 
same as of plants, and it may be justly stated that animal histology 
took its origin from the histology of plants. In 1667 Robert Hooke 
published the results of his microscopical investigations of a piece of cork. 
He found that it consisted of a number of small boxes or cells, and thus 
the name cell was introduced into histologic nomenclature. In 1838 
the botanist Schleiden published the results of his elaborate studies of 
plant structures, and established the fact that, no matter how widely 
individual plants and parts of plants differ from one another in their 
general appearance, in their minute structure they reveal themselves as 
constituting aggregations of cells, or modifications of such. He however 
laid particular stress upon the fact that each cell contains a substance, 
semifluid in nature, which constitutes the essential, life-carrying part 
of the cell, which by later investigators was named protoplasm. Further- 
more, he found that within the protoplasm there can always be distin- 
guished a kernel-like body, which was named nucleus. 



GENERAL CYTOLOGY; EMBRYOLOGY; AND HISTOGENESIS. 



2 95 



Inspired by personal contact with Schleiden, the anatomist Schwann 
undertook extensive examinations of various tissues of animal bodies. 
In 1839 he published the results of his investigations and established the 
fact, that no matter how widely various animals and organs of animals 
differ in regard to their general appearance, in their minute structure 
they always present aggregations of cells, or modifications of such. 

This conformity in the results of the investigations of Schleiden and 
Schwann has revealed the most interesting as well as the most important 
fact, that just as all chemical compounds have as their ultimate units 




Fee 240. — Cells from bulb of a fresh onion forming a membrane M, cell membrane; N, nu- 
cleus seen from the surface; Nn, nucleolus; V, vacuoles, X 240. (After Stirling.) 

the atoms or the molecules, so all living tissues have as their ultimate 
units the cells. This forms the foundation for all considerations of 
plant and animal histology up to the present time, notwithstanding 
the recent advancement in our knowledge of the structure and life of the 
cell itself. 

According to the views of Schleiden, Schwann and their contempo- 
raries, a cell consists of: (i) a cell- membrane; (2) a substance contained 
within — a protoplasm; and (3) a small kernel-like body — a nucleus. 

Continuous extensive studies of the structure of cells in various 
animal tissues have, however, very soon revealed the fact that while in 
all cells there can always be demonstrated a protoplasm and a nucleus, 
the presence of a cell-membrane is an exceedingly rare occurrence, and 
that this part therefore cannot be considered as an essential one in the 
structure of a cell. It was also found that there may be a number of 



296 HISTOLOGY. 

other structural elements present in one or the other kind of cells, but by 
their absence the individuality of the cell is by no means lost. The 
establishment of these facts naturally led to a change in the conception of 
the cell-structure, and in 1861 a distinguished investigator, Max Sclmltze, 
narrowed down the definition of a cell to a mass of protoplasm con- 
taining a nucleus, and this definition is generally accepted as the most 
appropriate one up to the present time. 

Another very important point pertaining to the conception of the 
cell has attracted the attention of all well-known investigators for a long 
time, namely, the origin of cells in general, and the formation of new cells 
in the different tissues for substituting old ones when they are worn out 
or lost through injury or disease. It was at one time believed that cells 
may originate spontaneously by transformation from organic or inorganic 
chemical compounds. By persistent investigations it was however 
gradually determined that the so-called spontaneous generation is much 
less frequent than had at first been supposed, and at present the belief 
in the formation of cells in that way is scarcely held by anybody. It is 
definitely established that all varieties of animal cells, no matter to which 
class of tissues they may belong, always originate from preexisting cells 
of the same kind. Furthermore, it was ascertained that while the 
different tissues of the animal body can be easily distinguished from one 
another in their mature state, there are great similarities between them 
in the early periods of the development of the animal. These similari- 
ties become the more pronounced the further back we trace them to 
their starting-point, and find that all tissues collectively take their origin 
from the same source, namely, from the ovum or egg, which, according 
to our knowledge at present time, is also nothing else but a single cell. 

The polymorphism of cells, or the great variety of cell-forms, which 
was observed in the animal body, has naturally suggested the assumption 
that the structure of cells with their essential and accessory parts 
is by no means a simple one, and further investigations have proved this 
to be correct. The study of cells and tissues has been greatly facilitated 
by the discovery of various substances, animal as well as vegetable, and 
particularly chemical compounds, which, when brought in contact with 
the tissues, show a distinct chemical affinity to the different parts of it 
and possess the ability of staining them in a distinct characteristic manner. 
Some of these substances stain the protoplasm of the cells and are 
therefore called protoplasmic stains, others stain the nucleus and are there- 
fore called nuclear stains. By application of these staining materials it 
was made possible to reveal the details in the structure of the various 



GENERAL CYTOLOGY; EMBRYOLOGY; AND HISTOGENESIS. 



297 



cells, and in the following we will give a description of the minute struc- 
ture of the cells, as it is generally accepted at present time, and may be 
observed in one or the other kind of physiologically differentiated cells 
during the various stages in their life-history. 

According to the statement made above, the essential parts of a 
cell are the protoplasm and the nucleus. The term protoplasm is how- 
ever used in modern histology to designate the substance of the whole 
cell, while the substance of the cell-body surrounding the nucleus is 
termed cytoplasm; the substance of the nucleus is sometimes termed 
nucleoplasm, but most frequently simply nucleus. We will also use these 
terms in our description. 



Nuclear membrane 



Achromatic 
substances 
of the nu- 
cleus 



Linin 



Nuclear sap 




Centrosome 



"-Inclusions 






Cuticular stratum — 



Filar-mass 



Interfilar-mass . 




Nuclein 
cords 



Netknots 



- -Nucleolus 



" Microsomes 



Chromatic 
substances 
of the nu- 
cleus 



- ■ Cell membrane (pellicula) 



Fig. 241. — Scheme of a Cell. Microsomes and filar-mass only partly sketched. (Stokr.) 



Cytoplasm. — The structure of the cytoplasm has been described 
differently by different investigators, and several theories have been 
advanced in that respect. Some have described it to be of an homo- 
geneous nature; others found it to be reticular; again, it was claimed to 
be granular; by some it is supposed to be foam-like. Thanks to experi- 
mental research-work carried out within recent years, it is positively 
known, at present, that the most important of all vital manifestations of 
the cells — metabolism — takes place in the cytoplasm, and this being the 
case the various stages of the process naturally manifest themselves in the 
structure of the cytoplasm. In this, and in the diversity of the methods 
used in preparing tissues for examination, must be sought the cause for 



298 HISTOLOGY. 

the difference in the results obtained. There is a general agreement 
among histologists, however, that even when fresh, unstained tissues are 
examined there can be recognized in the cytoplasm two different parts, 
one forming a reticulum or network and called spongioplasm; the other, 
a substance more homogeneous in character, which fills out the spaces 
of the network and is called hyaloplasm. The reticulum of the spongio- 
plasm may at times show a bead-like structure, thus presenting a granular 
appearance. The hyaloplasm may also be not entirely homogeneous, 
but contain very fine granules which are known as microsomes. There 
may be also found in the cytoplasm various foreign inclosures, such as 
particles of pigment, droplets of oil, some apparently empty spaces known 
as vacuoles, etc. All such inclosures are collectively known as meta-, 
para- or dculoplasm. There is also very often seen a condensation of the 
peripheral part of the cytoplasm, constituting what is called an ecto- or 
exoplasm, which without any sharp lines of demarcation passes into the 
more central part, called endoplasm. A cell-wall or -membrane must be 
considered as a highly specialized part of the exoplasm. 

Nucleus, — In the great majority of instances, the nucleus is a spheri- 
cal or oval body, situated usually either in the center of the cell or nearer 
to one pole of it than to the other, and varies in size generally in propor- 
tion with the size of the cell. In general its structure may be said to be 
somewhat similar to that of the cytoplasm, as it also consists of a reticu- 
lum or network, and a substance, more fluid in nature, filling the meshes 
of it. The network of the nucleus is however more complicated in its 
structure. By means of staining the specimens with so-called nuclear 
stains it has been revealed that the nuclear network consists of two 
different substances, first, of an exceedingly delicate, non-stainable reticu- 
lum, apparently very similar to the one of the cytoplasm, and called 
linin; secondly, of a substance which has the form of threads, or of a 
network, or of a mass of granules, and has the power of absorbing very 
actively certain dyes. This latter substance has therefore received the 
name chromatin substance in contradistinction to all other substances 
of the nucleus, which are not affected by the dyes, and therefore known 
as achromatin substance. 

The nucleus is separated from the cytoplasm by a distinct line of 
demarcation, which is known as nuclear membrane. This may however 
disappear at times, thus making possible an interchange of the substance 
of the nucleus with that of the cytoplasm. It generally takes place 
during the process of the so-called cell-division, of which we will speak 
later. 



VITAL MANIFESTATIONS OF CELLS. 299 

Nucleolus. — This is a small body found in the center of the nucleus 
in the great majority of cells. In some instances, as for example in nerve 
cells, it becomes very conspicuous. The significance of it in the cell- 
life is however not as yet well established. 

Centrosome. — This is a minute body observed in cells in close relation 
to the nucleus and generally situated just outside of the nuclear membrane. 
It consists of a more homogeneous substance and contains in its center a 
small dot, the centriole. The centrosome can not always be seen, but it 
becomes very conspicuous during a certain period of activity of the cell, 
namely, during cell-multiplication, when it plays a very significant role. 

Other constituents of cells have been mentioned already. 

VITAL MANIFESTATIONS OF CELLS. 

In animals consisting of but one cell, the so-called unicellular organ- 
ism, all life functions are naturally exerted by this one cell. The results 
of microscopic and experimental investigations of the last half century 
have however led to the conviction that the life activities of all the familiar 
higher animals, in health as well as in disease, are dependent on the 
activities of the cells of which they consist. It is for that reason that, 
while the study of the details of this subject belong to the domain of 
physiology, some of the more important points of it are also generally 
considered in connection with the description of cell-structure. The 
most conspicuous vital characteristics of cells are the following: 

Metabolism. — Of all the vital activities of cells, this is the most essen- 
tial one. It represents the sum total of all those physico-chemical proc- 
esses taking place in cells which have as their ultimate result the main- 
tainance of the life and individuality of the cells. It therefore embraces 
the process of taking into the body material suitable for its nutrition; 
the process of converting some of it into a part of its own substance to 
replace that which became worn out; transformation of another part 
into various kinds of energy; finally, the elimination of substances, not 
suitable for further use, as waste-products. 

Growth. — Newly-formed cells are generally not of the same size as 
mature cells, therefore a part of the substances assimilated by young cells 
is used up by them for the increase of their size. This goes hand in hand 
with the shaping of the cell until it becomes identical with the one from 
which it originated. 

Irritability. — This is a faculty of living cells to respond to various 
influences: mechanical, chemical, thermical, etc. It generally results in 



300 HISTOLOGY. 

those peculiar manifestations of the cells to which they are specially 
adapted, for example, secretion, motion, etc. 

Conductivity. — This is the ability of cells to convey and transmit 
impulses from one part of the cell to another and from one cell to another. 

Motion. — This is the property of cells to change their relative posi- 
tion to the surrounding media. It occurs in three different forms: 

(i) Amoeboid Motion. — Similarly to the amoeba, cells may send out thin 
or thick projections from their body, called pseudo podia, and if such pro- 
jections become attached to some foreign object and the rest of the body 
is drawn after it, there results a creeping motion. Cells endowed with 
such motion are known as wandering cells. In some instances such cells 
may, by means of their processes, surround foreign bodies and either 
incorporate them by assimilation into their own body-substance, or 
deposit them by expulsion into various other localities. Such wander- 
ing cells are known as phagocytes. 

(2) Ciliary Motion. — Some cells, usually those columnar in shape, 
have one of their free surfaces beset with a number of delicate, transparent, 
hair-like processes called cilia, which are constantly lashing forward 
and backward (reminding of the blades of grass in a field, when acted 
upon by a strong wind) producing a strong current, thus sweeping along 
various small substances. 

(3) Contractile Motion. — This is generally observed in fiber-shaped 
cells, such as are found in muscles. It consists of a shortening of the fibers 
in the direction of its long axis. The result is an approximation of the two 
ends of the fiber and consequently an approximation of the parts to 
which the two ends are attached. 

Reproduction. —While all the just mentioned cell-properties either 
serve to maintain their own individuality, or are the manifestations of 
their function in the animal economy, reproduction is the property of 
cells to form new cells similar to their own, a kind of rejuvenation, as it 
were. The object of this is either to substitute new cells for the ones 
which have become worn out and cast off, or, by adding new ones to their 
own ranks, increase their functional ability. 

The studies of the structural changes which take place in cells 
during cell-reproduction, have been pursued with great zeal and enthu- 
siasm during the last four or five decades, because it was through these 
studies that the true and complex structure of cells was revealed. We 
will therefore consider this phenomenon in its details at this point. 

The cell-structure, as described on page 298, is considered as the 
one which presents the cell in its so-called resting state, and the changes 



VITAL MANIFESTATIONS OF CELLS. 



3OI 



which have been observed in it during the process of reproduction are 
the following: 

In the first place, the centrosome, whether it has previously been visible 
or not, becomes very distinctly defined, and divides in two, which grad- 
ually separate from one another, and ultimately pass to opposite poles 



,nu. 





* . -SD. .' %'<;::VC * 
/--chr. ', ^fj^iPir 

* -:p - 





nu. 




..sp. 



Fig. 242. — Diagram of indirect or mitotic cell-division; A, cell with nucleus in resting stage; 
B, cell with nucleus in skein stage and with centrosomes separating; C, formation of chromo- 
somes; D, longitudinal splitting of the chromosomes; E, separation of the chromosomes; 
F, diaster stage ; G, H, formation of the two daughter cells; c, centrosome; cl, chromatin thread; 
chr., chromosomes; nu, nucleus; n, nucleolus; sp., nuclear spindle; w, cell wall. (After 
Calloway.) 

of the cell; they remain connected with each other however by means 
of finely drawn-out non-stainable fibers, known as the achromatic spindles. 
In the meantime the nuclear membrane disappears and the chroma- 
tin substance of the nucleus, whatever be its form during the resting 
stage, assumes the form of a continuous, densely coiled-up thread, which 



302 HISTOLOGY. 

is known, as the close skein. This coil gradually becomes in a measure 
entangled, and is then known as the loose skein. 

The next step is one of the most significant ones in the whole process. 
The whole chromatin thread becomes broken up into a number of seg- 
ments known as chromosomes, and it is a very important fact that, while 
the number of chromosomes varies in cells of animals of different species, 
there is always an equal number of them in cells of animals of the same 
species. The chromosomes become arranged in the equatorial plane 
of the spindle in a somewhat star-like fashion, and this is then spoken of 
as the monaster stage. Next, a splitting of each individual chromosome 
in its longitudinal direction takes place, and in this way the number of 
chromosomes becomes doubled, each one consisting now of a pair, or 
presenting a twin-chromosome, as it were. The two chromosomes of 
each pair gradually separate from one another and one of each pair ulti- 
mately passes to opposite poles of the spindle, near the centrosome. Here 
they also assume a star-like arrangement, and we have there what is 
known as the diaster -stage. The chromosomes are generally regarded at 
the present time as the bearers of heredity. 

The changes, which take place in each one of the stars, from now on 
present simply the reverse of those described above for the formation of 
the monaster, i.e., the individual chromosomes become united with one 
another, thus forming two skeins. At first the skeins are loose, then a 
condensation takes place and two close skeins are formed. Finally, 
around each one of them a membrane is formed, and there appear two 
nuclei similar to the one from which they originated. 

While this final shaping of the nuclei is going on, the cytoplasm of 
the cell becomes constricted half-way between the two nuclei, and when 
this is completed a division has thus taken place, and we have two cells 
instead of the original one. 

The various investigators have laid paticular stress on one or the 
other stage during the process of cell-multiplication. Accordingly, various 
names have been given to the same process, they all have their justifica- 
tion and are in use as synonyms. Taking into consideration the ultimate 
result of the process, it is called cell-division. In view of the fact that 
the changes are most conspicuously going on in the nucleus, it has been 
spoken of as karyokynesis. Because the chromatin substance of the 
nucleus assumes the form of a thread, the process is called mitosis. Finally, 
as this form of cell-division is accomplished in a rather complicated 
round-about way, it is also known as indirect cell-division. 

While the process of cell-division just described is the most frequently 



ORGANS AND TISSUES. 



3°3 



met with, there can be observed a more simple one, which leads to the 
same end. Without any preliminary rearrangement in its structure, the 
nucleus of the cell becomes elongated, then it assumes a dumb-bell shape, 
finally the connecting neck becomes broken across and thus two so-called 
daughter nuclei are formed which gradually separate from one another; 
in the meantime a constriction of the cytoplasm takes place, and as a 
result there appear two cells instead of one. For the reason that in 
this form of cell-division the nuclear substance does not assume any 
thread-like arrangement, and the end is accomplished in a rather simple 




Fig. 243. — Direct cell division (Amoeba). — A, active specimen with pseudopodia; B, becom- 
ing spherical preliminary to division; C, beginning of elongation and constriction; D, later 
stage; E, daughter cells forming pseudopodia; ec, exoplasm; en, endoplasm; /, food particle; 
n, nucleus; ps, pseudopodium; v, vacuole. (After Galloway.) 

manner, this process is called amitosis or direct cell-division. This 
process has been observed in the white corpuscles of the blood, occasion- 
ally in the liver, and not infrequently in pathologic conditions; it is assumed 
that this mostly occurs in cells having a lowered vitality, there is 
however much to be learned yet in regard to this question. 



ORGANS AND TISSUES. 

All through the animal kingdom, with the exception of the unicellular 
organisms, we observe, among the various elements of which they consist, 
the manifestation of a phenomenon commonly known as division of 
labor. We find groups of cells united together in various fashions to 
perform a certain function, and such aggregations of cells, specialized to 
fulfill well-defined duties in the economy of the organism, are called organs. 
Only among the lower animals, however, do we find that the organs consist 



304 HISTOLOGY. 

of only one kind of cell. Higher in the scale of animal organization, we 
find the organs exhibiting more and more complexity in their make-up. 
They consist of groups of various kinds of cells, and the individual groups 
present very little similarity to each other, sometimes becoming modified 
to such an extent that it is very difficult to recognize the cellular nature 
of them. While the name organ conveys the idea of a physiological unit, 
the texture of it is known as tissue. If the tissue presents an aggregation 
of elements, similar in character, we speak of simple tissues; and where 
we have an aggregation of elements of various kinds, we speak of complex 
tissues. 

To gain a proper understanding of the characteristic appearance of 
the various simple tissues, their various modifications, combinations and 
transformations in forming that multitude of structures, which is ob- 
served in a mature organism, it is .necessary to trace them to their first 
beginning, or histogenesis. 

We have stated elsewhere that an animal body takes its origin from 
the egg, ovum, which is nothing else but a single cell. While ova of 
various types of animals differ in size, they all have relatively the same 
structure and are all specialized for the same purpose — to produce a new 
individual. The changes which an ovum undergoes in course of its 
development into a new individual are in all cases principally the same, 
and therefore the knowledge gained from the study of one form serves us, 
in a general way, to understand all others. The study of these consecu- 
tive changes constitutes the subject of General Embryology and 
Histogenesis. 

General Embryology and Histogenesis. 

In all animals with sexual mode of reproduction, the cell from 
which the whole organism ultimately develops is itself a product of 
the union of two highly differentiated and specialized cells; one, supplied 
by the male individual and formed in the testicle — the spermatozoon; the 
other, supplied by the female individual and formed in the ovary — the 
ovum. These two so-called elements of reproduction are derived by trans- 
formation of special cells in the body, the so-called germinal cells, which 
are set aside, as it were, for the purpose to be eliminated from the body 
at certain periods and used as a foundation for the propagation of the 
species. The process of entrance of the spermatozoon into the ovum, 
and the ultimate union and fusion of the two elements is known as 
the fertilization. Before this fertilization can take place, however, the 



GENERAL EMBRYOLOGY AND HISTOGENESIS. 305 

ovum has to undergo a preliminary process called maturation, which 
consists in the elimination of half of the quantity of its nuclear substance 
in form of two small bodies, expulsed at one of the poles of the ovum, 
and therefore called polar bodies. 1 The fertilized ovum is that cell from 
which a new being gradually develops; and while the mode of develop- 
ment varies somewhat in its details in different species of animals, the 
general principles of the process always remain the same. In the follow- 
ing we endeavor to give a short general account of the essential points 




Fig. 244. — Four stages in the maturation and fertilization of the ovum (partly diagram- 
matic). A, formation of the polar bodies and entrance of the spermatozoon; B, the male and 
female pronuclei; C, nuclei coming together; D, pronuclei uniting to form segmentation 
nucleus; e.n., egg nucleus; p. 6., polar bodies; s, spermatozoon; s. c, sperm centrosome; 
s.n., sperm nucleus; 5. e., segmentation nucleus produced by the union. (After Galloway.) 

in the development of the ovum, but for the details of it we must refer 
to the various text-books on embryology. 

After a short period of rest, the fertilized ovum begins to undergo 
the so-called process cleavage or segmentation. By means of karyo- 
kinesis it divides into two halves, producing two cells. Each of these cells 
in turn again divides, giving rise to jour cells, and this is succeeded 
by another division, forming eight cells, and by repeated division of this 
kind there arises a solid mass of smaller cells called morula or mulberry 
mass, from its resemblance to a berry. As the cells increase in number, 
the mass also increases in size by the absorption of nutriment, and a 

1 By expulsion of the polar bodies the number of chromosomes in the ovum is reduced 
to one-half of it; the fusion with the spermatozoon, which is also supplied only with half the 
number of chromosomes, the normal quantity is restored and the fertilized ovum thus con- 
tains heredity bearing substance from both its progenitors. 
20 



3° 6 



HISTOLOGY. 



gradual arrangement of the cells in a definite fashion takes place, varying 
however with the character of the ovum. They may arrange themselves 








H I 

Fig. 245. — Diagrams showing the development of the germ layers. (After van Beneden 
and St oh r.) A, Two-celled stage; B, four-celled stage; C, morula stage; D to /.transverse 
section of various stages of the blastoderm. 



in the form of a layer of cells spherically surrounding the so-called seg- 
mentation cavity; or they may be spherically arranged around a mass of 
yolk; or they may form a disk-like arrangement, floating as it were on 



GENERAL EMBRYOLOGY AND HISTOGENESIS. 307 

a spherical mass of yolk. In any case we observe the cells to be cylin- 
drical in shape and, lying side by side in regular fashion, form a some- 
what skin-like arrangement. This is therefore called the blastoderm, 
which means a germinal skin, and the individual cells are named blasto- 
meres. The continuous multiplication of the cells of the blastoderm 
results in either an infolding (invagination) or a splitting off (delami- 
nation) of some of the cells, which by their own gradual multiplication 
form a distinct continuous layer of cells beneath the first one. It is 
then generally spoken of as a two-layered blastoderm, of which the 
outer layer is called ectoderm or epiblast, and the inner layer entoderm 
.or hypoblast. These two layers are called the primary germinal 
layers, and the cells of which they consist show great similarities in 
many respects. Very soon, however, some of the cells, on the surfaces 
of the two primary layers which face each other, detach themselves 
therefrom by the process of splitting off or delamination and lodge in 
the gradually widening space between the layers. As the number of 
these cells increases also through their own multiplication, it very soon 
becomes possible to distinctly recognize a third layer of cells, which, on 
account of its being situated between the other two, has received the 
name of middle layer or mesoderm or mesoblast. With the formation 
of the mesoderm, the blastoderm is said to consist of three germinal layers, 
and with this the foundation for the development of the various tissues 
and organs is finally established. The resemblance of the cells consti- 
tuting the different layers gradually becomes more and more lost, and 
differentiation then begins to take place. Through all the consecutive 
changes, the derivatives of the ectoderm and entoderm retain the charac- 
teristic tendency to be arranged in groups of cells lying closely side by 
side in a simple or stratified fashion. The derivatives of the mesoderm 
are, on the contrary, characterized by a looser arrangement of their cells, 
some of which remain connected with one another by means of proto- 
plasmic prolongations or processes, while others retain the faculty of 
wandering away and intermingling with the derivatives of the other two 
layers. Each of the three layers gives rise to well-defined structures 
adapted to distinct physiological functions, but none of them can in this 
respect be substituted by another, without producing abnormal conditions 
in the organism. 



CHAPTER II. 

Elementary Tissues: Epithelial Tissue, Connective Tissue, 
Muscular Tissue, Nervous Tissue, Blood and Lymph. 

The differentiation of the three layers of the blastoderm into various 
tissues goes on parallel with the development of the body in general, and 
with the growing complexity of organization along the scale of the animal 




cnt- - 



u B 



Fig. 246. — A, Diagram of a longitudinal section through the body of a Hydra; it presents 
the structure of an animal, in which the walls consist of only two layers of cells specialized to 
perform all the function. B, a small portion of the wall more highly magnified. (After 
Galloway.) 

kingdom, the number of varieties of tissues as well as their complexity 
also increases. On page 294 we have already indicated, however, that 
we can resolve all the various tissues into five well-characterized typical 
groups, which are known as elementary tissues. These are: (1) epithelial 

308 



EPITHELIAL TISSUE. 309 

tissue, (2) connective tissue, (3) muscular tissue, (4) nervous tissue, 
(5) blood and lymph. 

Among the lower animals there are some which resemble to a 
considerable extent the early embryonic stages of higher animals. 
Their walls consist of two layers of cells, which resemble one another 
very much, and are transformations respectively from the ectoderm and 
entoderm. They are endowed with the ability to perform all functions 
which characterize living bodies and represent that typical group of cells 
known as epithelial tissue. Accordingly, in the higher animals, also, the 
principal functionating parts of the various organs, including the nervous 
system, are by various transformations derived from the ectoderm or 
entoderm, and are either epithelial tissue proper or highly specialized 
modifications of it. 

The mesoderm begins to develop when the other two layers are 
already well differentiated, and, accordingly, the tissues which take 
their origin from that layer are found very sparingly, or not at all, in 
lower animals, but become more and more conspicuous as we ascend in 
the complexity of animal organization. It gives rise mainly to tissues 
which, while not constituting the principal parts in the texture of various 
organs, are nevertheless of very great importance to them and form indis- 
pensable auxiliary parts of them. These are the connective tissues, 
muscular tissue and blood and lymph. Some epithelial formations take 
their origin in the mesoderm also, but their number is very limited. 

We will now consider the individual tissues in detail. 

I. EPITHELIAL TISSUE. 

Epithelial tissue is found as coverings of all surfaces of the body, 
those directly exposed to the air as well as those which form variously- 
shaped cavities and communicate with the air indirectly through narrow 
or wide openings. The cells composing the epithelial tissue are known as 
epithelial cells. These are definite in outline, show very clearly a cytoplasm 
and a nucleus, and lie side by side in a regular fashion. There is just 
enough substance — so-called intercellular substance — between the individual 
cells to hold the cells together. Three principal forms of cells are gener- 
ally met with: (1) the flattened or squamous, (2) the cylindrical or columnar, 
and (3) the many-sided or polyhedral. If a single layer of cells only is 
present, it is known as simple epithelium; if there are two or more super- 
posed layers of cells present, it is spoken of as stratified epithelium. In 
simple squamous epithelium the cells are flattened or scaly and the 



3IO HISTOLOGY. 

nuclei are round and also flattened. In the stratified squamous variety, 
which is the one most frequently met with, only the superficial layers are 
squamous, while the deeper ones are more irregular, and may gradually 
become columnar. 

In columnar epithelium there is quite a variety in the outline of the 
cells. In simple columnar epithelium the cells may be either long — 
high columnar — or of a medium size — cuboidal — or very short- — low colum- 
nar. They may be beset on their free surfaces with numerous minute 
hair-like processes, which are constantly vibrating during life and are 
known then as ciliated epithelium. In the stratified variety also the 
superficial layer only may be typically columnar, while the others may be 
of a different shape. Among a continuous superficial layer of columnar 
epithelium, there may be found scattered here and there cells which have 
somewhat the shape of a conical cup and contain in their cytoplasm mucus 
in various states of formation. From time to time a contraction of their 
cell-body takes place, and their content, which is of a mucous character, 
is poured out upon the surface. Owing to their shape, these cells are 
called goblet cells. 

The polyhedral epithelium may be found in various localities in the 
body. If the body of the cells contains some pigment granules, it is 
known as pigmented epithelium. When polyhedral or columnar epithe- 
lium is forming the constituting part of the so-called glandular organs, it 
is known as glandular epithelium. A very conspicuous variety of epithe- 
lial cells of various shapes is represented in the so-called neuro-e pilhelium , 
which is specialized for the creation and perception of the special senses. 

Epithelial tissue never contains blood-vessels; their nutrition takes 
place by the absorption of nutritive juices through the clefts between the 
cells, or the cement-substance. It is obvious that when a stratified 
epithelium consists of a large number of layers, the superficial ones may 
receive very little nourishment or none at all, which accounts for the 
constant exfoliation of cells from the surface skin and other parts. 

II. CONNECTIVE TISSUE. 

Connective tissue is the most widely distributed tissue in the animal 
body. It holds the individual parts of other tissues together; it connects 
the various tissues with one another, and at the same time keeps them 
separated from one another; it gives firmness to the body as a whole, and 
a support for the various organs within it; it forms variously constructed 
channels for the distribution of the nutritive material to the various parts 



CONNECTIVE TISSUE. 



3 11 



of the body. With the variety of functions just enumerated there is a 
corresponding variety of forms in which connective tissue is found repre- 
sented in the body. 

We have stated above that connective tissue takes its origin from 
the mesoderm. With the advance in the differentiation of the ectoderm 
and entoderm the derivatives of these two layers gradually become 
separated from one another, and the changes of the mesoderm follows 
closely in their steps. It becomes split into two secondary layers, one of 
which attaches itself to the derivatives of the ectoderm, ready to serve 
them with the above-enumerated functions, and is called parietal meso- 
derm, while the other plays the same role in regard to the derivatives of 




~ect. 




-mesi 



Fig. 247. — Diagrams of transverse sections through the body of an embryo of a vertebrate. 
It shows the relation of the three germinal layers, ect., Ectoderm; ent., entoderm; mes., 
mesoderm; coe., coelom or body cavity; sk., beginning of skeleton; n., beginning of spinal 
cord; g, lumen of the gut. (After Galloway.) 



the entoderm, and is called visceral mesoderm. The gap, which remains 
between these two secondary layers, is known as the body-cavity or 
coelom. The characteristic feature of the mesoderm tissue is that the 
cells do not lie side by side and form continuous layers, but are separated 
from one another, sometimes quite considerably, and the spaces between 
the cells are occupied by a substance somewhat gelatinous in consistency 
which is called intercellular substance and is obviously a product of the 
cells themselves. The cells are stellated or spindle-shaped, and their 
thinned-out processes unite and interlace with one another, forming a 
network. The formation of the various forms of connective tissue from 
the mesoderm in course of development is due mainly to the differentia- 



312 HISTOLOGY. 

tion and various chemical changes which that intercellular substance 
or matrix undergoes. In view of the fact that the intercellular substance 
constitutes the predominating part in connective tissue, it is the fea- 
ture upon which the classification of this tissue is based, and we can dis- 
tinguish five well-defined characteristic groups: (i) mucous or embryonic 
connective tissue, (2) ordinary or fibrous connective tissue, (3) cartilage, 
(4) bone, (5) dentin. 

Mucous or Embryonic Connective Tissue. — This tissue closely 
resembles in its structure the mesoderm tissue in earliest stages of the 
embryo, as described above. It consists of a semi-gelatinous, mucoid 
matrix, and within it are scattered stellate or spindle-shaped cells, and 
here and there thin fibers. The latter present mainly the elongated and 
anastomosing processes of the cells, some of them however are un 
doubtedly products of the matrix itself, by a process that reminds some- 
what of coagulation. This tissue is found at birth in the umbilical cord, 
as the so-called jelly of Wharton; in the adult human body it is found in 
the pulp of the teeth and in the vitreous humor of the eye. 

Ordinary or Fibrous Connective Tissue. — This tissue is present in 
the skin and mucous membrane, in the intermuscular tissues, in tendons, 
in fascia and aponeuroses, and in the tissues connecting various organs. 
It is composed of a meshwork of fine fibers of two kinds. The first, 
which makes up the greater part of the tissue, is formed of very fine, 
•white, structureless fibers arranged closely in bundles and bands crossing 
and intersecting in all directions. The second variety, or the yellow, 
elastic fiber, has a much sharper and darker outline, not arranged in 
bundles, but is intimately mingled with the white fibers by twisting 
around and among its filaments. These are known as the elementary 
connective-tissue fibers. The size of the connective-tissue bundles 
depends upon the number of elementary fibers present, and by a variation 
in the arrangement of the bundles variety in the character of the fibro- 
connective tissue is produced in different localities. When the fibrous 
connective tissue is formed into an unbroken mass, as in mucous mem- 
brane, the minute bundles are collected into smaller or larger groups 
(the trabecules:), and these are in turn associated into groups. In the skin 
and mucous and serous membranes, the trabecular of the connective- 
tissue bundles are separated, and, by crossing and recrossing one another, 
form a dense, fan-like structure. In other tissues, as the tendons and 
fascia, the bundles are arranged in parallel layers. In the submucous 
tissues the connective-tissue fibers are loosely woven, the fibers crossing 
and intermingling, with the intervening spaces unusually large, resulting 



CARTILAGE. 313 

in a loose, flabby tissue. Two varieties of fibrous connective tissue are 
distinguished — namely, (a) Compact or fibrillar connective tissue, form- 
ing bands of either white fibers, or yellow elastic fibers, or mixed fibers; 
and (b) loose or areolar connective, forming a network or reticulum. 
Modifications of the loose variety are found represented in adipose tissue 
and lymphoid tissue. 

The fibrous connective-tissue cells are few in number, of several 
varieties, and variously shaped, being flattened, stellate, or apparently 
distorted by pressure from surrounding cells or fibrous bundles. In 
the mucous membrane the cells are oblong and somewhat flattened, 
having many branches which reach out and, uniting with like processes 
from neighboring cells, form a network. Other connective-tissue cells 
are comparatively larger, oval or rounded in form, granular in appearance, 
rich in protoplasm, and are known as plasma-cells. The body of connect- 
ive-tissue cells, besides containing a nucleus, frequently contains pigment- 
granules; these are known as pigment-cells. These are seldom found in 
mucous or serous membranes, being principally confined to the integ- 
ument. Fat-globules may also be found in fibrous connective tissue, and 
when of considerable size unite and form a rounded cell, called a fat-cell. 
Numerous fat-cells uniting, and well supplied with blood-vessels and 
nerves, form adipose tissue, or fat. Fat-cells are frequently found in 
areolar tissue as well. When fibrous connective tissue is immediately 
contiguous to epithelium, it becomes somewhat modified and a new 
membrane is formed, called the basement membrane, or membrana propria. 
This membrane is a thin, transparent, structureless layer, and, when in 
connection with those mucous membranes provided with a layer of vas- 
cular fibrocellular tissue, may appear as the formative substance out of 
which successive layers of epithelial cells are generated. In the ducts 
and glands — for example, the salivary glands — the basement membrane 
forms the proper walls of the tubes, and the cells here generated, and 
corresponding to the epithelial cells of the coarser mucous membranes, 
are known as gland-cells, rather than epithelial cells. This, however, 
is a distinction without a perceptible difference, the location and function 
as secreting cells being alike in each. 

Cartilage. — Cartilage is a semi-opaque, non-vascular tissue, white 
in color, and composed of a matrix containing nucleated cells. The 
matrix is somewhat elastic and rather dense. The cells are simple in 
form, being spheric or slightly inclined to angularity. The variation in 
the character of cartilage is due rather to the difference in the character 
of the matrix than to the cellular structure, the principal variation in the 



314 HISTOLOGY. 

cells being in their size. The cells lie in the spaces or lacunae of the 
matrix, which they completely fill. Investing the free surface of most 
cartilaginous tissue (articular cartilage excepted) is a thin but tough 
and firm fibrous membrane — the perichondrium. This membrane is well 
supplied with blood-vessels and nerves, and is essential to the growth and 
maintenance of the cartilage. There are three varieties of cartilage — 
namely, hyaline cartilage, elastic carti'age, and fibrocartilage. 

Hyaline cartilage is of a faint pearly-blue color, slightly transparent, 
and is found investing the articular ends of the bones — for example, 
the condyles of the mandible; also forming the costal and nasal carti- 
lages, as well as those of the trachea, bronchi, and a part of the larynx. 
Hyaline cartilage is distinguished by a granular or homogeneous matrix. 
The cells, which contain a nucleus with nucleoli, are usually grouped 
together in patches, and are somewhat irregular in outline, appearing 
flattened near the free surface of the tissue in which they are placed, and 
inclined to be perpendicular to the surface in the more deeply-seated 
portions. The matrix is dimly granular in appearance, resembling 
ground glass, and receiving its name from this fact. That part of the 
cartilage close to the perichondrium is supplied with cells much smaller 
than those occupying the lacunae in the substance of the mass, and the 
growth of the cartilage is most active in this part. Lining each lacuna 
is a delicate membrane (the capsule), winch primarily is but partly filled 
out, but as the cell or cells increase in size, this membrane is carried to 
the walls of the lacuna. Articular hyaline cartilage is non-vascular, 
being nourished by the blood-vessels of the bone beneath. 

Elastic cartilage is of a dull-yellow color, and is sometimes called 
yellow cartilage. It is not present in the mouth, but occurs in the exter- 
nal ear, in the epiglottis, and in part of the larynx. Its structural compo- 
sition is quite similar to hyaline cartilage, but may be distinguished 
from it by a network of fine elastic fibers which penetrate the matrix. 
The cells are rounded or oval, containing nuclei and nucleoli. 

Fibrocartilage is yellowish or milky white in color, and is much more 
widely distributed throughout the body than the elastic variety. It is 
present in the temporomandibular articulation. Like those previously 
described, it is composed of cells and a matrix, the latter being made up 
of fibrous connective tissue arranged in bundles, and for this reason it 
is scarcely deserving the name of cartilage, only that in other portions 
continuous with it cartilage-cells may be found in abundance. Between 
the strata of the fibrous bundles are numerous nucleated cells, which are 
oval and more or less flattened, and each enveloped in a delicate capsule. 



BONE. 



315 



Cartilage is further classified into two divisions — temporary and 
permanent — the former term being applied to that kind of cartilage which 
in the fetus and in youth is destined to be converted into bone (for example, 
Meckel's cartilage); the latter class including all those cartilages which 
are generated as such, and continue to serve in that capacity. Temporary 
cartilage closely resembles the hyaline variety, being formed of a matrix 
in the lacunae of which the cells are located. These cells, however, are not 
grouped together as in hyaline cartilage, but are more uniformly 
distributed throughout the matrix. 

Bone. — Bone is mainly composed of tricalcium phosphate and 
cartilage. The matrix of osseous tissue has a distinguishing feature 



Calcified 
Matrix 




Center of Calcifica- 
tion 



Enlarged 
Lacuna? 



Hyaline 
Cartilage 



Fig. 248. — Developing Bone. X 40 



produced by the blending of organic and inorganic substances, resulting 
in hardness, solidity, and elasticity. The combination of organic and 
inorganic elements in bone is of such a nature that either part may be 
removed without destroying the other. The matrix is' composed of the 
salts of lime, especially calcium phosphate, and of slender fibrils united 
by a cement-substance into bundles of various sizes. The cement-sub- 
stance is chiefly composed of insoluble lime-salts, principally carbonates 
and phosphates. These two kinds of structure are found to be present 
in different parts of the same bone, forming a dense or compact, and a. 



316 HISTOLOGY. 

spongy or cancellated tissue. The former occur in the shaft of long 
bones and in the outer layer of flat or irregularly formed bones. Cancel- 
lated bone-substance occurs in the extremities of the long bones and in the 
interior of flat and irregular bones. The irregularly formed maxillary 
bones give place to both kinds of bony structure; the external layer of 
the superior maxillae and the body and rami of the inferior maxilla are 
composed of compact tissue, while the interior of these bones and the 
condyloid processes of the mandible are spongy or cancellated in their 
nature. When examined by the microscope the bony substance is found 




nc 






\ 



Concentric flSL_ 

Lamella' ~ r r- *■ _«. .^ " -»*» «."" " *■ - % 

f •-- ;<* . - ■- > ^\yc > - - » 

Haversian M| >. ,~ \ ( C-* < ""* '•» \ » 

Canal M| - -■ fc , v ^ _ . ) ^5 , '.-•.>• ? Haversian 

*, ' • . ' * v' - ".v ' -* Canal 




• * - 



Fig. 249. — Transverse Section through Shaft of Long Bone. X 30. 

occupied by numerous little spindle-shaped spaces — lacunce. Branching 
out from these in various directions are minute canals — canaliculi — 
which anastomose with similar canals from neighboring lacunae. In the 
maxillary bones no other canals than these may be visible, but if a 
transverse section be cut through one of the long bones, an additional 
space makes its appearance (Fig. 249). 

These spaces are known as the Haversian canals. They are circular 
in outline and appear as a center for a small, circular district mapped out 
by concentric layers, the lacunae and canaliculi following the same con- 
centric plan, and through each other communicating with the Haver- 
sian canals. The general direction of the Haversian canals is longitu- 



BONE. 



317 



dinal with the long axis of the long bones, and in the flat or irregular- 
shaped bones they are somewhat irregular in formation and ramify in 
various directions. In the osseous matrix each lacuna contains a bone- 
cell. These are nucleated, protoplasmic cells. In developing bone, 
these cells, which do not completely fill the lacunae, are connected by 
numerous branches or processes passing through the canaliculi; in older 
bone very few processes are observed. 

There are two processes by which bone may be prepared for histo- 
logical examination, by one method which results in the destruction of 




> ' ' A 

\ ' \ • \ \ 

x * \ \ \ * \ 




Lacuna 



Haversian ^k> 1 £ 

Canal \ \ *t " "• % V^\ \ ' 

. M * '. A ' \ U • V 

Fig. 250. — Longitudinal Section of Long Bone. X 30. 

the organic elements, or by another which removes the inorganic ele- 
ments. In the former process the organic matter is removed by simply 
drying the structures, after which thin sections may be prepared and care- 
fully examined under the microscope, when the Haversian canals, lacunae, 
and canaliculi will be seen forming a complete concentric network. In 
the latter method the inorganic substance is removed by immersing a 
fresh bone in dilute picric acid, C 6 H 2 (N0 2 ) 3 OH, which readily decalcifies 
it, and when properly prepared sections are placed under the microscope 
the organic contents of the lacunae and canaliculi alone are visible. 

The concentric laminae of bone is riveted together by numerous 
delicate rods of processes named Sharpey's fibers, these delicate fibers 
passing through the laminae to perform this office. 



318 HISTOLOGY. 

Periosteum and Bone-marrow. — The interstices of spongy 
bone are filled, with a soft mass — the bone-marrow — and the external 
surface of the bone is covered by a fibrous membrane — the periosteum. 
This membrane is absent where bones are joined to each other by liga- 
ment or cartilage, and over articular surfaces. The periosteum is a 
compact connective-tissue membrane. It consists of two layers: an 
outer, fibrous layer rich in blood-vessels, which forms the connection with 
adjacent structures; an inner or osteogenetic layer containing few blood- 
vessels, loose in texture, but rich in elastic fibers and spheric connective- 
tissue cells, with oval nuclei. These are the formative cells of bone and 
are called osteoblasts. These cells appear in the lower strata of the inner 
layer, or the layer in contact with the bone, and are especially numerous 
during the period of development. Through the blood-vessels of the 
bone the marrow, internally, is placed in communication with the perios- 
teum externally; small branches given off from the numerous arteries and 
veins of the periosteum enter the Haversian canals, upon which they pass 
to the canaliculi, thus communicating with the blood-vessels of the 
marrow. In like manner numerous nerves enter the substance of the 
bone, first passing into the Haversian canals, after which they become 
closely associated with the minute blood-vessels and are distributed 
to the periosteum and bone-marrow. The bone-marrow, besides fill- 
ing the interstices of the spongy substance, is also found occupying 
the central cavity of long bones, and in the larger Haversian ca- 
nals. The marrow is of two varieties, distinguished by its color, being 
either red or yellow. Red marrow is found in the flat bones (including 
the maxillae), the vertebras, and ribs, while yellow marrow occurs in the 
long bones of the extremities. Red marrow is composed of a delicate 
connective-tissue network supporting, besides the marrow-cells, a few 
fat-cells and giant-cells. In the long bones the yellow marrow is sur- 
rounded by a connective-tissue membrane lining the medullary canals. 
Marrow-cells and giant-cells are present in abundance. Marrow is very 
vascular and contains many osteoblasts. 

Dentin. — This structure, as well as cementum, which in many 
particulars closely resembles bone, will be fully considered in connection 
with the histology of the tissues of the teeth. 

III. MUSCULAR TISSUE. 

Muscular tissue consists of elongated or fiber-cells and according to 
the structure of these fibers it is divided into three classes — non- striated, 
striated and cardiac. 



NON-STRIATED MUSCULAR TISSUE. 



3 J 9 



Non-striated, Smooth, or Involuntary Muscular Tissue. — This 
tissue consists of contractile fiber-cells which are elongated, spindle- 
shaped, and cylindric, with exceedingly elongated extremities, which 
become shorter and thicker through contraction. They are quite variable 
in length (i/io to 1/450 of an inch), and are composed of a pale, homo- 
geneous-looking protoplasm, each inclosing an elongated or rod-shaped 
nucleus, which is flattened if the cell is so formed. The muscular fibers 
are firmly bound together by a cement-substance, forming fasciculi, 
which in turn are collected into strata or membranes, which may be dis- 




FiG. 251. — Transverse Section of Striated or Voluntary Muscular Tissue. X 40. 



posed parallel, or crossing and recrossing, forming an intricate network. 
The connective-tissue septa provide a passageway for the larger blood- 
vessels, while the capillaries penetrate the fasciculi forming a compli- 
cated network with oblong meshes. Involuntary muscular tissue is not 
found in the mouth except in the ducts of the salivary glands. They form 
the main constituent part of the middle layer or coat of blood-vessels 
and are particularly abundant in arteries. 

Striated or Voluntary Muscular Tissue. — Striated muscular 
tissue is composed of long, cylindric fibers, which are regularly transversely 
striated. In most instances their extremities are attached to bones by 
means of tendons, as, for example, the cheek- and lip-muscles. The 



3 20 



HISTOLOGY. 



fibers are grouped together by fibrous connective tissue into various sized 
bundles, forming fasciculi. There is much variation in the length of the 
fibers composing the fasciculi in different muscles. In most instances 
the fasciculi which serve to make up the bundles of a single muscle con- 
tinue parallel with one another throughout their length. Surrounding the 
whole muscle is a layer of connective tissue called epimysium; this pene- 
trates between the individual bundles and forms a covering for each one 
of them, which is called the perimysium, and passing from this into the 
substance of the bundle is a still more delicate connective tissue, the 




Fig. 252. — Striated or Voluntary Muscular Tissue. X 40 



endomysium, which separates the individual fibers from one another. 
The former structure carries the larger blood-vessels and nerve-fibers, 
while the latter supports the capillaries. 

Each muscular bundle may again be divided into smaller bundles, 
which in turn are ensheathed in a similar manner and further divisible, 
so continuing, until the primitive fasciculi, or so-called muscular fiber, 
is reached. Striped muscular fiber consists of a structureless, elastic 
sheath, the sarcolemma, which structure represents the cell- membrane, 
and closely invests a number of filaments or fibrils. Besides the fibrillar, 
there is contained within this fine, structureless, transparent membrane 



STRIATED OR VOLUNTARY MUSCULAR TISSUE. 32 1 

the sarcoplasm, a faintly granular substance resembling protoplasm, 
but not identical with it. This substance serves in the capacity of a 
matrix for the fibrillae. The nucleus is found beneath the sarcolemma. 
The fibrillae are arranged parallel to one another, being supported by 
the sarcoplasm. It will thus be seen that each fiber of a striated 
muscle comprises the sarcolemma, the muscle-nuclei, the fibrillae, and, 
finally, the sarcoplasm, filling all the interstices, first between the 
fibrillae of each muscle-column, between the columns of each group, 
and between the groups themselves. The disposition of the sarcoplasm 
may be most favorably studied by a cross-section through the fibers, 
appearing as a delicate but clear network, within the meshes of which 
are the muscle-columns. Striated muscular fibers are usually tapering 
off and becoming thinner toward their extremities. In rare instances 
they are branched at their ends. This condition is present in the 
tongue, the extremities of the fibers passing transversely into the oral 
mucous membrane, where they become further subdivided. The striated 
or voluntary muscles make up the muscular tissues of the lips, cheeks, 
tongue, and soft palate. 

Cardiac or Involuntary and Striated Muscular Tissue. — This 
tissue is found only in the heart. The fibers are short, striated and have 
the nucleus in the center. 

IV. NERVOUS TISSUES. 

Until within recent times it has been stated that the nervous tissue 
consists of two histologic elements known as nerve-cell and nerve-fiber; 
that these two elements differed not only in their mode of origin, but in 
their structure and physiologic endowments. At the present time it 
is believed that the entire nervous system consists of an infinite number 
of definite independent morphologic units, which, through having a com- 
mon origin and a similarity of structure, have, nevertheless, different 
functions in different parts of the body. This neurologic unit has been 
termed the neuron, and, as represented schematically in figure 253, may be 
said to consist of: First, the nerve-cell, or neurocyte; second, nerve- 
process, or axon; third, the end-tufts, or terminal branches. Each of 
these three main portions of the neuron presents a variety of secondary 
features which are related to their functional activities. 

The Nerve-cells, or Neurocyte. — The nerve-cells are found in the 
cortex of the brain, in the interior of the spinal cord, in the various gang- 
lia of the cerebrospinal and sympathetic nervous systems, and in the 
organs of special sense. All neurocytes are the modified descendants of 
21 



322 



HISTOLOGY. 



Nerve- 
cell 



^Dendrites 



Naked 
Axis- 
cylin- 
der 



Axis- 
cylin- 
der En- 
veloped __, 

by- 
Medul- 
lary 
Sub- 
stance 



Collateral Branch 



independent oval or pear-shaped cells (the neuroblasts), originating from 

the epithelial cells which form the 
medullary tube. The neurocyte is 
at first smooth, devoid of processes, 
and endowed with ameboid move- 
ment. In the course of develop- 
ment the cells project a greater or 
less number of processes and as- 
sume a variety of shapes and sizes, 
in accordance with variations in 
functions; thus, the cells may be 
spheroid, pyramidal, spindle- 
shaped, stellate, etc. The body of 
the cell consists of a protoplasmic 
basis, more or less granular, con- 
taining a well-defined nucleus and 
nucleolus. A centrosoma has also 
been found in the nerve-cell in 
many situations. There is no evi- 
dence, however, of the existence of 
a cell-membrane. From the body 
of the neurocyte there arises one 
or more protoplasmic processes, 
which, passing outward in various 
directions, divide and subdivide 
into a greater or less number of 
branches, which are collectively 
known as dendrites or dendrons. 
The ultimate subdivisions and 
terminations of a dendrite, though 
forming an intricate feltwork, 
always end free, never anastomos- 
ing with one another. Arising from 
the cell-body, the dendrites resemble 
in appearance and structure the 
cell-protoplasm, or cytoplasm. In 
the cortex of the cerebrum and in 
the cortex of the cerebellum the 
dendrites are characterized by 



Axis- 
cylin- 
der En- 
veloped 
by both 
Neuril- 
emma " 
and the \ 
Medul- 
lary 
Sub- 
stance 




.'Terminal Branches 



Fig. 253. — Diagram of a Neuron. 
Stohr's "Histology.") 



(Frctn 



short, lateral projections known 



NERVOUS TISSUES. 323 

as lateral buds or gemmules, which impart to the dendrite a feathery 
appearance. 

The Axon, or Nerve-process. — The axon is the first outgrowth of the 
protoplasm of the neuroblast, but with the development of the neurocyte 
it becomes so differentiated from the dendrites that it can be readily 
distinguished from them. It usually arises from a cone-shaped projection 
of the cell-body, though occasionally it arises from a dendrite itself. It is 
characterized by a short, regular outline and a hyaline appearance. The 
majority of the cells, especially in the mammalia, possess but one axon, 
though in the developing ganglion-cells of the spinal nerves two distinct 
axons are present. In their subsequent development the two axons appear 
to blend together to form but a single axon, which, at a short distance from 
the cell, again divides into two branches, which pursue opposite directions, 
one passing directly into the spinal cord, the other toward the periphery. 
The axon may continue as an individual structure for an indefinite dis- 
tance, varying from a few millimeters to ioo cm. In the former instance 
the axon, at a distance of a few millimeters from the cell, breaks up into 
a number of branches, which form an intricate feltwork in the neighbor- 
hood of the cell. This type of cell is not widely distributed, being 
confined largely to the cerebellum. In its course the axon, more espe- 
cially in the central nervous system, gives off a number of side-branches or 
collaterals, which do not differ from the axon itself, either in structure or 
appearance. The axon of the peripheral nerves, especially the spinal 
nerves, are devoid of collaterals throughout their extent, except, perhaps, 
in the immediate neighborhood of the cell. The more or less elongated 
axon becomes inclosed at a short distance form the cell with a thick 
layer of fatty material, forming a medulla or myelin, inclosed by a deli- 
cate cellular sheath (the neurilemma), and thus constitutes what is com- 
monly known as a medullated nerve-fiber. In the central nervous sys- 
tem the neurilemma is frequently wanting. In the sympathetic system 
the myelin is wanting, though the axon is inclosed by a delicate sheath 
resembling the neurilemma, thus constituting a non-medullated nerve- 
fiber. The collateral branches are provided with similiar investments. 

The End Tufts, or Arborizations. — Each axon, as it approaches 
its final termination, breaks up into a number of branches, which vary 
in complexity and appearance in different regions. They are always 
free from any medullary investment, and appear to be formed by the 
splitting of the axon into a number of fine filaments, which remain inde- 
pendent of one another. In peripheral organs, as muscles, glands, and 
blood-vessels, the tufts are in direct organic connection. In the cen 



3 2 4 



HISTOLOGY. 



tral nervous system the end-tufts are in more or less intimate relation 
with the dendrites of other neurons. 

Nerves. — Nerves are to be regarded, therefore, as groups of axons, 
with their medullary investments connecting the peripheral organ with 
the central nervous system. 

The nerves are arranged in two great systems — the cerebrospinal 
and the sympathetic. In the cerebrospinal nerves the conducting 
media — the nerve fibers — are arranged in parallel or interlacing bundles, 
and these are further grouped into nerve-branches or nerve-trunks. 
The bundles are connected by intervening fibrous connective tissue 



Shea' 



Endoneuriur 



Medullan 
Sheath 







Perineurium 




Fig 254. — Portion of Transverse Section of Human Median Nerve. X 200. 



(the epineurium), and through this tissue the principal blood-vessels 
ramify to supply the nerve-trunks, together with a plexus of lymphatics 
and numerous fat-cells and plasma-cells. 

The size of the nerve-bundles, or funiculi, is regulated according 
to the size and number of nerve-fibers which they contain. Investing 
each funiculus, or primary bundle of nerve-fibers, is a connective-tissue 
sheath — the perineurium. The fibers composing this sheath are arranged 
in lamellae, being separated from one another by lymph-spaces vari- 
able in size, through which communication is afforded the lymphatics of 
the epineurium. Within the bundles the nerve-fibers are held together 



NERVES. 



3 2 5 



by fibrous connective-tissue — the endoneurium. The epineurium holds 
together and envelops the several funiculi of the nerve-trunk, the peri- 
neurium investing each funiculus, or primary bundle of nerve-fibers, 
and the endoneurium extending among and around the individual fibers. 
Nerve-fibers are divided into two classes — which classification is depen- 
dent upon the presence or absence of a medullary sheath or covering — 
into the medullated or white, and the non-medullated or gray. The 
medullary sheath, or white substance of Schwann, is a bright, fatty sub- 
stance (the myelin) surrounding the axon, or axis-cylinder, the conducting 
or central part of a nerve-fiber. Between the medullary sheath and the 
axis-cylinder there is present a small amount of albuminous fluid. Closely 







rv- • 

Perineurium ' .',i/y^s> -*' J?* 



Fig. 255. — Transverse Section, Bundles of Nerve-fibers, Human Median Nerve. X 30. 



surrounding the medullary sheath, and forming the outer boundary 
of the nerve-fiber, is the neurilemma, or sheath of Schwann. Between 
this delicate, structureless membrane and the medulla there are placed 
at intervals oblong nuclei, surrounded by protoplasm; these are the 
nerve-corpuscles. Besides the division of nerve-fiber into medullated 
and non-medullated, each division is susceptible of further subdivision, 
dependent upon the presence or absence of the neurilemma. Non- 
medullated nerve-fibers without a neurilemma are composed of an axis- 
cylinder only; they are cylindric or band-like in form, transparent, and 
show faint, longitudinal striations. Non-medullated nerve-fibers with a 
neurilemma are composed of an axis-cylinder surrounded by a neurilemma, 
and are homogeneous throughout their extent. 



326 HISTOLOGY. 

Medullated nerve-fibers are those which are partly, but never entirely, 
invested by a medullary sheath. They may or may not possess a 
neurilemma; in the former instance they consist of an axis-cylinder and a 
medullary sheath only. The axis-cylinder, or essential part of the nerve- 
fiber, is cylindric or band-like, occasionally exhibiting a delicate, longitu- 
dinal striation, which appearance is due to its being composed of a primi- 
tive fibrilke. 

The nerve-cells or ganglia-cells are found in the ganglia as well as 
along the course of the nerves. They are composed of granular or 
faintly striated protoplasm, inclosing a characteristic nucleus within which 
is a nucleolus. They differ greatly in form as well as in size, the spheric, 
spindle-shaped, and irregularly stellate forms being the most common. 
In the latter numerous processes are given off, forming the stellate out- 
lines. The cells are variously named, according to the number of proc- 
esses. If one process is present, the cell is termed a unipolar cell; if 
two, a bipolar; and if a number of processes exist, they are named multi- 
polar. The processes are of two varieties — the axis-cylinder process 
and the branched protoplasmic process. The various forms are most 
readily distinguished in the multipolar cells. The axis-cylinder process 
is readily characterized by its hyaline appearance and unbroken outline. 
The protoplasmic processes are thicker, granular, and striated. 

V. BLOOD AND LYMPH. 

It is rather difficult to conceive that blood and lymph can be spoken 
of as representing an animal tissue. The conception of a tissue generally 
carries with it the idea of some well-defined, stationary texture; blood 
and lymph, on the contrary, present fluids, which uninterruptedly stream 
along within closed channels — the blood- and lymph-vessels. A 
closer study of the development and structure of blood and lymph, and a 
little reflection in regard to their physiological role in the economy, shows, 
however, that, while they present certain peculiarities, it is nevertheless 
justified to classify them as a tissue. 

Development. — Blood and lymph, as well as blood- and lymph- 
vessels, originate in the mesoblast. Some of the cells of the latter become 
more closely united in groups arranged in form of cords. These gradually 
become transformed into tubes, the walls of which consist of a single 
layer of cells arranged in epithelial-like fashion, and are called endothe- 
lium. Other cells, while remaining separated from one another, become 
inclosed within the channels and give rise to the red and white blood- 



BLOOD AND LYMPH. 327 

corpuscles. The intercellular substance between them, in the meantime, 
becomes differentiated as a fluid called blood- plasm, thus allowing the cells 
free floating within the channels. 

Structure. — The microscopic examination reveals the fact that 
blood and lymph consists of a large number of cells. The cells do not 
however lie side by side and are held together as in epithelial tissue; they 
are neither separated from one another, and at the same time fixed in a 
relative position to each other by means of processes and a matrix of 
various consistency, like connective tissue. They are suspended in a 
substance which normally is a fluid, but when however examined in a 
state of coagulation, it has a more gelatinous consistency and contains 
fibers, thus reminding of the intercellular substance in connective tissues. 

Physiology. — Blood and lymph presents the medium which carries 
nutrient and building material to all parts of the body, and in exchange 
receives and carries away the waste products. To fulfill this function, 
their active constituents, the cells, must be supplied with facilities for 
reaching all parts of the body, and this is obviously accomplished through 
the intercellular substance being a fluid. 



CHAPTER III. 

The Mucous Membrane of the Mouth; of the Lips; of the Cheeks; 
of the Gums; of the Roof of the Mouth, Hard and Soft Palate ; 
of the Floor of the Mouth; the Tongue. 

HISTOLOGY OF THE TISSUES OF THE MOUTH. 

Mucous Membrane of the Mouth. The mucous membrane lining 
the cavity of the mouth consi>ts of two parts — the epithelium and the 
tunica propria; beneath the latter, and forming the deeper part of the 
mucous membrane, is the submucosa. 



r 



- 



^fli 




i 




layer 
of Cells 



Infant Lay- 
er of Cells 



Connective 
Tissue 



Fig. 256. — Vertii .1] S( 1 lion, Mucous Membrane of the Mouth, Human Embryo. X 150. 



The epithelium of the mouth is a thick, stratified, squamous epithe- 
lium, the most superficial cells being scale-like or horn-like. The cells 
are arranged similar to those in the epiderm, the lower layers are 
columnar in form, and contain very little pigment. 

328 



MUCOUS MEMBRANE OF THE LIPS. 329 

The tunica propria is a somewhat dense feltwork of interlacing connect- 
ive-tissue bundles, interspersed with elastic fibers. The tunica propria 
penetrates the epithelium in the form of cylindric or conic papilla?, which 
differ in length with the variation in the thickness of the epithelium. 
As the mucosa is usually thickest in the lips, gums, soft palate, and uvula, 
accordingly the papilla? are of the greatest length in these parts. The 
tunica propria passes into the submucosa so gradually that a positive line 
of demarcation cannot be established. 

The submucosa consists of a bundle of fibrous connective tissue with 
but few elastic fibers. This structure is somewhat loose in texture and 
is loosely attached to the underlying periosteum. Over the major portion 
of the gums and the entire hard palate the submucosa is attached to the 
bones of the mouth through the medium of their periosteal covering. 
It is in this loosely constructed tissue that the glands of the mucous mem- 
brane are situated. These are for the most part branched, tubular, mucous 
glands. Besides adipose tissue in the form of groups of fat-cells, striped 
muscular tissue is present in the submucosa. In some parts of the mouth 
this tissue forms a conspicuous portion — namely, in the sphincter muscle 
of the lips (orbicularis oris) ; also in the soft palate, uvula, and pillars of 
the fauces. 

The blood-supply to the mucous membrane of the mouth is principally 
distributed in two systems, the larger vessels to the submucosa and the 
capillaries to the tunica propria. The larger vessels break up and send 
a dense network of capillaries through its substance and to the numerous 
papilla? which extend into the epithelium. Numerous veins ramify 
through the superficial part of the tunica propria. The lymphatics form 
two networks, the submucosa giving place to the coarser vessels, while 
the fine parts are distributed to the tunica propria. 

Nerve-supply to the Mucous Membrane of the Mouth. — In the sub- 
mucosa the medullated nerve-fibers form a wide-meshed reticulum, from 
which numerous primitive fibrilla? pass to the tunica propria, where they 
terminate or continue as non-medullated nerve-fibers, and penetrate the 
papilla? of the epithelium, forming networks. 

Mucous Membrane of the Lips. — Beginning as a direct continuation 
of the integument or external covering of the lips, the labial covering, 
including the integument, may be divided into three parts — namely, a 
cutaneous portion (best described in this connection), a transitory portion, 
and a mucomembranous portion. 

The cutaneous portion, covered by a thin epidermis, consists of a 
double layer of somewhat flattened epithelium. Immediately beneath 



33© HISTOLOGY. 

this is a thin, cellular, mucous layer, the cells composing it being spheroid 
in form, and containing nuclei which are proportionately large. Sub- 
jacent to this is the cutis, composed of fasciculi of fibers intersecting 
and closely woven together, the principal fibers passing toward the free 
border of the mucous membrane covering the contiguous surface of the 
lip. These fibers are for the most part connective-tissue fibers, intermingled 
with elastic-tissue fibers. Numerous small, vascular papillae are found 
upon the surface of the cutis; these are cylindric or conic in form, and 
project for some distance into the rete mucosa — the lower layers of 
living cells of the epidermis. Equally distributed at various depths in 
this tissue are numerous hair- and sebaceous follicles. The general 
direction of the hair-follicles in the upper lip is downward, while those 
occupying the lower lip are turned upward. Other than the distinction 
noted by the difference in color of the parts, the cutaneous portion may 
be distinguished from the transitional mucous membrane by the absence 
of hair-follicles and sebaceous glands in the latter. 

The transitional portion of the mucous membrane of the lips is out- 
lined externally by the outer border of the red portion of the lips, and 
internally by that prominent part of the labial convexity which comes in 
contact with the opposing labial fold, leaving the transitional portion 
exposed to view when the lips are in occlusion. The epithelial layer 
of this surface does not begin where the hair-follicles cease to exist, a 
slight interspace appearing upon the cutaneous portion which is devoid 
of these follicles. At its line of beginning the transitional portion of the 
labial mucous membrane is quite thin, but rapidly increases in thickness 
in passing toward the mucomembranous portion. Superficially the 
cells are much flattened, closely associated with one another, and devoid of 
nuclei. The cells of the middle and deeper layers are oblong or spheric, 
and provided with irregularly shaped nuclei. The chief fibrous tissues 
of the transitional portion, which are thinnest at the point where the 
hair-follicles cease to exist, are united into flexiform fasciculi, which are 
separated at various points to give passage to numerous minute blood- 
vessels. The fibrous tissues increase in thickness as the mucomembran- 
ous portion is approached. Numerous thin and somewhat elongated 
papillae are distributed over the surface of the transitional portion. 

The mucomembranous portion of the mucous membrane of the lips 
includes all that portion covering the labial folds within the mouth, 
beginning at the line of occlusion on the contiguous surface and extend- 
ing to the gums. The epithelium is much thicker than that previously 
described, and presents the characteristic layers common to stratified, 



MUCOUS MEMBRANE OF THE CHEEKS. 



33 1 



squamous epithelium. Superficially the cells are flattened and tubular, 
provided with nuclei of similar form. In the middle layer the cells are 
flattened and oblong, followed in the deeper layer by irregularly formed 
nucleated cells. A variety of fibers make up the structure— one class 
fine in texture and united into fasciculi, intermingled with elastic fibers, 
together with another set of coarse, strongly looped fibers. Whenever 
the fibers of the tunica propria assume a definite general direction, they 
are horizontal, passing from right to left and encircling the oral aperture. 
The tunica propria is beset with numerous conic papillae which project 



Embryonal 

Mucous 
Membrane 





Buccal Cavity 



) 



/ 



Fig. 257. — Vertical Transverse Section through Head of Human Embryo, about the Sixth 
Week, showing Single Buccal Cavity. X 30. 

into the epithelium; these are longest where the epithelium is thickest. 
The mucous membrane forming the labial frena is covered by an epithelial 
layer which is much thinner than that distributed to other parts of the lips. 
The fibers in these are irregularly distributed, and the papillae are small 
and not so numerous. The coronary arteries and their accompanying 
veins course through the lips near the junction of the transitional with 
the mucomembranous portion of the mucous membrane. 

Mucous Membrane of the Cheeks. — The mucous membrane of 
the cheeks presents but little variation in its structure from that of the 
mucomembranous portion of the labial mucous membrane. The buccal 
epithelium is the same in structure and thickness as that of the lips, 



33 2 HISTOLOGY. 

excepting the disposition of cells in the middle layer, where they are greater 
in number and more closely associated, being somewhat distorted by con- 
tact. The papillae, which project from the mucosa into the epithelium, 
are somewhat broad at their base, with elongated extremities, the height 
of which is quite variable, in some instances penetrating well into the 
epithelium, at others merely entering its deeper layer. At the anterior 
portion of the cheek, or that in the region of the angle of the mouth, 




Fig 258. — Section through the Mucous Membrane of the Cheek, showing the Papilla; of 

the Mucosa in Transverse Section. 

the mucous membrane, by its submucous portion, is in immediate contact 
with the fibers of the buccinator muscle, and throughout the entire surface 
of the cheek it is closely associated with this muscle. The membrana 
propria is dense immediately beneath the epithelium, but as the buccinator 
is approached it becomes much less so. 

Mucous Membrane of the Gums. — The mucous membrane cover- 
ing these parts is, on account of the numerous tendinous fasciculi which 
enter into its construction, extremely dense and tough, these character- 
istics being more strongly manifest here than in any other portion of the 



MUCOUS MEMBRANE OF THE GUMS. 



333 



oral mucous membrane. These qualities are especially pronounced 
about the gingival margins and over the major portion of the 
alveolar walls, being closely bound down to the bone by direct pro- 
longations of the tendinous fasciculi of the periosteum which penetrate 
the membrane. As the gingival mucous membrane passes into that of 
the lips and cheeks it gradually becomes less dense. The epithelium 
of the mucous membrane of the gums is composed of lamina of tessellated 




Epithe- 
lium 



Papilla 



Fig. 259. — Section through the Gums, showing Epithelium and Basement Membrane. 



and ribbed cells. The superficial cells are the flattened cells of pave- 
ment epithelium; subjacent to this they become thicker and deeply ribbed, 
while the deepest cells are conic or cylindric with conic extremities. The 
tissue composing the tunica propria is made up of flattened fasciculi of 
connective tissue, the fibers of which run parallel with one another. 
Numerous elastic fibers are also present. Three sets of fibers are to be 
distinguished in the mucous membrane of the gums — those which run 
vertically, those which pass in a horizontal direction, and those which 
radiate or are distributed fan-like. Of the first named, the fibers extend 



334 HISTOLOGY. 

from above downward; in the second class they pass from right to left 
parallel with the surface; the third class, including those fibers which are 
reflected from the alveolodental membrane, are distributed in fasciculi 
about the margins of the alveoli. 

Mucous Membrane of the Roof of the Mouth. — Hard Palate.— 
The mucous membrane covering the hard palate is, in very many respects, 
dissimilar to that surrounding the necks of the teeth and forming the 
palatogingival margins. Like the mucous membrane of the gums, that 
overlying the hard palate is dense and tough. The papilla? of the tunica 
propria, which penetrate the epithelium, are not so numerous as those 
upon the gums. In the posterior third of the hard palate they are some- 
what more numerous and generally a little more prominent than those in 
the anterior portion. In the median raphe and over the rugae, the papilla? 
are especially sparingly distributed. The epithelium is of the pavement 
variety, somewhat thinner in front than behind, the cells being more 
freely distributed at some points than at others. The mucous membrane 
of the hard palate is less in thickness anteriorly than posteriorly. The 
distribution of the fibers is such that they radiate from the alveolar borders 
toward the center of the palate, the anterior fibers passing obliquely 
backward, while those from the lateral walls pass parallel with one 
another to the median line. For the most part the fibers are broad and 
form a plexus between the epithelium and the submucous tissue. The 
submucous tissue is sparingly distributed over the central portion of the 
hard palate, but laterally is somewhat more abundant, containing a few 
fat-cells. 

Soft Palate, Uvula, and Fauces. — Passing backward from the posterior 
margin of the hard palate, the mucous membrane overlies the fibrous 
aponeurosis of the soft palate and its median and lateral prolongations — 
the uvula and pillars of the fauces. The epithelium is of the laminated 
pavement variety, with the deeper cells larger than those placed super- 
ficially. The substance of the mucous membrane is composed of fasciculi 
of connective tissue, intermingled with a plexus of elastic fibers. The 
fibers are distributed in three principal directions — from side to side, 
or horizontally, longitudinally, and obliquely. The oblique fibers are 
instrumental in forming the submucous tissue of both the soft palate and 
uvula. Numerous conic papillae project from the tunica propria into the 
epithelium; these are larger and more numerous on the uvula than on the 
soft palate. The tunica propria is somewhat variable in thickness to 
accommodate the glands, which are more or less numerous, and present 
in greater numbers in one instance than in another. In general the 



MUCOUS MEMBRANE OF THE FLOOR OF THE MOUTH. 



335 



membrane as a whole is thinnest along the margin of the hard palate, 
gradually increasing in thickness as the free border is approached. The 
folds of mucous membrane forming the pillars of the fauces present no 
peculiarity differing from that of the soft palate, save a more generous 
supply of elastic fibers. 

Mucous Membrane of the Floor of the Mouth. 

The Tongue.— The entire unattached surface of the tongue is covered 
by a reflection of the mucous membrane of the floor of the mouth. In this 
organ the general structure of the mucous membrane does not vary from 
that of other oral mucous membrane, being composed of an epithelium, 
a tunica propria, and a submucosa. The mucous membrane covering 



Epithelium 



Tunica 
Propria 







!iV 



Fig. 260. — Longitudinal Section through Mucous Membrane of the Human Tongue. X 20. 

the dorsum of the tongue presents special characteristics, which differ 
from that of the under surface and the floor of the mouth in general. 
In the former location the papillary elevations of the tunica propria are 
conspicuously developed, and with their covering of stratified, scaly 
epithelium cause the peculiar furred appearance. Three classes of 
papillae are distinguished, named, in accordance with their form, filiform 
papillae, fungiform papillae, and circumvallate papillae. 

The filiform papilla, which are very numerous over the entire dorsum 
and sides of the tongue, are conic and frequently prolonged into numerous 
horn-like processes, known as secondary papillae. As elevations from 
the tunica propria they are composed of well-defined fibrillated tissue, 



336 



HISTOLOGY. 



intermingled with numerous elastic fibers. The pavement epithelial cells 
are found overlapping one another, and provided with processes which 
project beyond the papilla?. 

The fungiform papillce are also distributed over the entire dorsum 
and sides of the tongue, but are somewhat less numerous than the filiform 
variety. They appear as well-defined elevations, and are connected 
with the tunica propria by a constricted portion or neck. The entire 
free or rounded surface of these papilla? is beset with secondary papillae. 
The epithelium is slightly thinner than that over the filiform papillae, this 
being the principal distinguishing feature. The numerous capillaries 
produce a rich red color, plainly observable through the transparent 




Fig. 261. — Longitudinal Section of the Mucous Membrane of the Human Tongue, showing 
the Fungiform and Secondary Papillae. X 80. 



epithelium. Connective-tissue bundles make up the bulk of these 
papillae, few elastic fibers being present. 

The circumvallate papillce are placed on the posterior portion of the 
dorsum of the tongue, and are few in number (eight to sixteen) . They are 
much larger than those already described, and in general resemble modified 
fungiform papilla?. They are flattened and broad, and differ from the 
fungiform by having a circular furrow or wall surrounding them. Second- 
ary papillae are present on the free surface only, the sides, and in some 
instances the walls surrounding them, being occupied by the end organs 
of the special sense of taste — the taste-buds. Other taste-buds are found 
upon the lateral margins of the tongue posteriorly, nestled in a group of 



BLOOD-SUPPLY TO THE MUCOUS MEMBRANE OF THE MOUTH. 337 

parallel folds of mucous membrane — the papillae foliata. The connective 
tissue within these papillae is similar to that in the fungiform papillae. 
On other parts of the tongue, or those portions not occupied by these 
specially constructed papillae, the epithelium is similar to that in other 
parts of the mouth. The tunica propria is less in thickness in and about 
the tip of the tongue, and is intimately connected with the subjacent 
muscular structure. As the root of the organ is approached, the tunica 
propria becomes thicker and more dense. The submucosa is especially 




Fig. 262. — Section through Epithelium, Near the Tip of the Tongue. X 40. 

intimately connected with the underlying parts at the margins and tip 
of the tongue. The extreme portion of the root of the tongue has its 
mucous membrane particularly modified by a special aggregation of 
adenoid tissue — developed lymph-nodules. These are large and readily 
perceptible to the naked eye. They are provided with a central opening, 
which dips down into a well-defined vault or crypt, which is lined by a 
reflection of the stratified oral epithelium. 

Blood-supply to the Mucous Membrane of the Mouth. — The oral 
mucous membrane derives its supply of blood from numerous branches 
of the external carotid artery — namely, the superior and inferior coro- 
nary, buccal, lingual, transverse facial, pterygopalatine, and the alveolar. 
Entering the submucosa, the minute terminal branches of these arteries 
are distributed parallel to the surface, and by anastomosis form plexuses 
from which other minute branches are given off to supply the papillae of 
the tunica propria. After coursing through the papillae the blood is 
22 



338 HISTOLOGY. 

discharged into a similar venous plexus, and thus conveyed from the 
parts. In a like manner the mucous membrane and papilke of the 
tongue are supplied, branches of the lingual artery conveying the blood 
to the parts. The dorsalis linguae supplies the mucous membrane of the 
dorsum of the tongue and pillars of the fauces, while the ranine artery 
by its minute branches supplies the remaining mucous membrane. Each 
papilla is entered by two or more arterial terminals, which divide, anas- 
tomose, and finally send off capillary branches to the secondary papilla?. 
Nerve-supply to the Mucous Membrane of the Mouth. — The 
distribution of the nerve-fibers to the oral mucous membrane is approxi- 
mately similar in all parts. The fibers, which are of the medullated 
variety, are first distributed to the submucosa, forming a wide-meshed 
reticulum. From this fibers are given off to the tunica propria, terminat- 
ing in end-bulbs, or, after losing their medullary sheath, are distributed 
to the epithelium, where their free extremities lie between the epithelial 
cells. The nerves of the mucous membrane of the tongue (the glosso- 
pharyngeal and lingual branch of the fifth) may have their endings similar 
to those in other parts of the mouth, or they may be intimately associated 
with the taste-buds. 

Literature. 

Legros and Magitot, 1880. 

Klein, "Structure of the Oral Lips," 1868. 

Sebastian, "Anatomy and Physiology of the Labial Glands," 1842. 

Kolliker, "Mikroskopische Anatomic" 

Kirke, "Physiology." 

Strieker, "Human and Comparative Histology." 

Stohr, "Text-book of Histology," 1896. 



CHAPTER IV. 

Glands and Ducts of the Mouth; of the Lips; of the Cheeks; of 
the Hard and Soft Palates; of the Tongue. — The Salivary 
Glands. 

GLANDS AND DUCTS. 

Glands of the Mouth. — The glands of the mouth, like the glands 
of other parts of the body, are composed almost entirely of epithelium, 
and may, therefore, be classed with the epithelial tissues. Glands exist 
in two principal forms — tubular and saccular (alveolar). The former 
occur either singly or in groups, and are further subdivided into simple 
tubular and compound tubular glands. A like condition is present in 
the saccular glands and similar terms are employed to qualify them — 
simple saccular glands and compound saccular glands. 

A simple tubular gland is one composed mainly of a simple tube- 
like structure; a compound tubular gland is one composed of a number of 
smaller tubes emptying into a single duct. 

A simple saccular gland is one formed by a sacculation of serous or 
mucous membrane into a single, simple sac, or by branched saccules 
having an excretory duct (alveolar system) ; a compound saccular gland is 
composed of a combination of branched saccules. 

In the larger glands a sheath is formed by the surrounding connective 
tissue, from which numerous septa are given off to the interior of the 
gland, dividing it into compartments varying in size. These are known 
as gland-lobules. The connective-tissue walls of the gland-lobules carry 
the larger blood-vessels and nerves. Most glands are divided into two 
essential parts — the gland-follicle and the excretory duct — the former 
being specialized for the secretory function, while the latter, by com- 
municating with the surface, conveys the secreted substance to that point. 

The gland-follicles are composed of a layer of gland-cells, usually 
simple in character, surrounding the follicular walls. External to these 
is a specially modified connective tissue, forming the basement membrane, 
or membrana propria. The appearance of the gland-cells and their 
nuclei is continually changing, being thus influenced by their functional 
activity. 

339 



34° 



HISTOLOGY. 



The excretory ducts consist of a wall of connective tissue and elastic 
fibers, lined by a columnar epithelium, either simple or stratified. In 
some instances the arrangement of the excretory ducts is much compli- 
cated, being divided into secretory tubes, which in turn are subdivided 
into smaller tubules — intercalated tubes. 




Fig. 263. — Section through the Glandular Tissue of the Tongue, x 40. 



The Glands of the Lips (Labial Glands). — The glands of the lips are 
situated in the submucosa, and arc first observed immediately within the 
line of labial occlusion, at which point the thickness of the epithelium 
becomes somewhat definite and general. These glands are variable in 
size, but all are sufficiently large to be observed without the aid of the 
microscope. They are of the compound tubular variety, and communi- 
cate with the surface through an excretory duct, which throughout the 
greater part of its extent is lined with stratified, scaly epithelium. In 
passing from the surface toward the gland-follicle, the main duct takes a 
spiral course obliquely through the tunica propria, and upon reaching 
the submucosa gives off numerous branches and twigs which terminate 
in the individual acini. The larger branches from the main duct are 



THE GLANDS OF THE CHEEKS. 34.I 

lined with stratified squamous epithelium, while the smaller twigs are 
provided with columnar epithelium. In many instances the main 
excretory duct, in its passage through the tunica propria, receives the 
principal duct from small accessory ducts. The framework of the labial 
glands is formed by the flexiform tissue composed of fasciculi of the fine 
connective-tissue fibers belonging to the submucous layer, together with 
delicate, coiled elastic fibers. This framework gives support to a minute 
system of capillaries and small nerve-fibers supplying the acini. The 
acini are so arranged that those belonging to a large duct are united 
into a lobule by the submucous connective-tissue fasciculi, and these in 
turn are formed into lobes. By a continuation of the same fasciculi and 
fibers which limit a lobe, and in the meshes of which the acini are situated, 
a sheath to the excretory duct is formed. Besides the branched, tubular, 
mucous glands of the lips, there are occasionally found, at the edges of 
the lips, sebaceous glands. 

The Glands of the Cheeks (Buccal Glands, Molar Glands). — The 
glands of the cheek are also situated in the submucous layer of the mucous 
membrane. They, like the labial glands, are of the compound tubular 
variety, and when microscopically examined are found to be similar in 
structure. They are somewhat larger than the labial glands and pro- 
portionately less numerous. The chief duct from each of these glands 
usually opens with a narrow mouth on the surface of the oral mucous 
membrane, and in its passage through the tunica propria takes a vertical 
or oblique direction. In the submucosa the chief duct branches into 
two or more smaller ducts, taking up alveoli. As the buccal glands are 
somewhat larger than the glands of the lips, they are composed of a greater 
number of ducts and alveoli. 

The Glands of the Hard and Soft Palate (Palatal Glands).— The 
mucous glands of the hard palate are situated in the submucosa and 
closely associated with the periosteum. They are compound tubular 
glands, and in all essential particulars are similar to the labial and buccal 
glands. They are quite numerous (200 to 300), isolated in the anterior 
portion, but are grouped into a single row or into two rows posteriorly. 
The glands are freely distributed in each lateral half, but are absent at 
the median line. 

In the soft palate the glands are of the same character, somewhat 
variable in size, the largest being found in the uvula. The excretory 
ducts from these glands vary in diameter, in the nature of their fibrous 
structure, and in the direction taken in passing to the surface. Over 
the surface of the soft palate the mouths of these ducts are represented 



342 



HISTOLOGY. 



by minute orifices slightly smaller than the body of the duct, but in the 
uvula the opposite condition is present, the mouth of the duct being wider 
than the body. The^course taken by the excretory duct is seldom a 
direct one, but after receiving all tributary branches passes obliquely 
through the tunica propria, and before entering the epithelium turns 
at an abrupt angle, and so continues until the surface is reached. The 
ducts are lined by a simple columnar epithelium, which in some instances 
is ciliated; the walls of the tubes consist of gland-cells and a structure- 
less membrana propria. In some instances the surface epithelium may 
be reflected for a short distance and partly serve in the capacity of a lining 
to the tubular walls. 

The Glands of the Tongue (Lingual Glands). — In this organ two 
varieties of glands are found, occurring both in the mucous mem- 




Fig. 264. — Section through Base of Tongue, showing Serous (ilands. 



brane and in the superficial muscular strata, being principally distin- 
guished by the nature of their secretions. The gland-cells of the one set 
are mucigenous, secreting mucin; these are the. mucous glands. The 
other set is productive of a serous fluid, thin, watery, and containing 
albumin; these are the serous glands. 

The mucous glands of the tongue are found along the lateral margins 
and over the root of the organ, being most numerous in the latter situation. 



THE SALIVARY GLANDS. 343 

They are of the compound tubular variety, and in most particulars are 
identical with the mucous glands of other parts of the oral cavity. The 
ducts are lined with ciliated columnar epithelium, and the walls of the 
duct consist of a homogeneous membrana propria and gland-cells. 
The glands occupying the root of the tongue are frequently 'found with 
their excretory ducts opening into the follicular crypts. The tubules 
consist of a structureless membrana propria and numerous gland-cells, 
the latter varying in appearance according to their function or functional 
activity. The crypts of the follicles constitute reservoirs for the acinous 
glands, and these receptacles frequently extend for some distance beneath 
the surface, receiving at various points the main excretory ducts from 
the mucous glands. These saccular-like reservoirs are lined by a well- 
defined capsule surrounded by a fibrous sheath, internal to which is an 
epithelial covering, a prolongation of the common epithelium of the 
mouth. Between these two layers are a number of minute, closed lymph- 
follicles placed in a single layer. The mucous glands on the lateral walls 
of the tongue are, for the most part, situated near the middle or pos- 
terior portion. The ducts from these glands usually open directly toward 
the cheek, but in rare instances they pass obliquely downward and open 
near the proper floor of the mouth. At the tip of the tongue, buried be 
neath the mucous membrane and some of the muscular fibers, may be 
found a pair of mucous glands (Nuhn's) which open by free orifices on the 
under surface. At the root of the tongue, flat, lenticulated elevations of 
the mucous membrane are present, beneath which is imbedded con- 
globate, glandular substance. These show a central orifice leading to a 
small pit lined with tessellated epithelium. 

The serous glands of the tongue are compound tubular glands, and are 
found in the region of the circumvallate papillae, closely associated with 
the taste-buds. The excretory ducts, lined with a simple or stratified 
columnar epithelium, the latter sometimes ciliated, open near the base of 
the papilla, or between the papilla and its wall. The tubules are similar 
to those in the mucous glands, consisting of a delicate, structureless mem- 
brana propria and gland-cells. The gland-cells are composed of a frail, 
transparent protopasm, containing rounded nuclei. 

The Salivary Glands. — The parotid, submaxillary, and sublingual 
glands each consists of an excretory duct, branching frequently in a tree- 
like manner into smaller ducts, lined throughout with a layer of epithelial 
cells. From the smaller ducts terminal branches are given off, which in 
turn are lined with epithelium. The other portions of the glands are 
invested by columnar epithelium, and arranged like grapes about the 



344 HISTOLOGY. 

main excretory duct, and consequently belong to the group of racemose 
glands. The terminal branches or alveoli attached to the smaller excre- 
tory ducts are so numerous that they become much compressed from 
pressure, and the grape-like appearance is more or less destroyed, and 
but little space is left for interstitial tissue. Each gland is inclosed in a 
fibrous connective-tissue capsule, and from this numerous septa of fibrous 
trabecular pass to the interior and divide the glandular substance, first into 
lobes, these being subdivided into lobules, the lobules by further sub- 
division forming the alveoli. The glandular connective tissue is loose 
in texture, containing many elastic fibers and lymphoid cells. Fine 
bundles of fibrous tissue, together with branched connective-tissue cor- 
puscles, constitute the connective-tissue matrix between the alveoli. 

The Ducts. — Entering the interior of the gland, the chief duct divides 
into a number of large branches, one of which passes to each lobe, each 
of these giving off several branches which connect with the several lobules. 
Upon close examination the central tube of each lobule is observed to 
throw off several small tubes — the intralobular tubes. Following these 
are the intermediate tubules, which continue into the terminal compart- 
ments. The chief excretory duct consists of a double layer of cylindric 
epithelium and fibro-elastic cartilage. Close beneath the epithelium is a 
compact membrana propria. The intralobular tubes are each provided 
with a distinct lumen. The walls are composed of a membrana propria 
lined by a layer of columnar epithelium, the cells of which contain a 
central round nucleus. 

The Parotid Gland. — The distinguishing histologic feature in this 
gland is found in its excretory duct (Stenson's duct), which is provided 
with a membrana propria, especially broad and compact, placed imme- 
diately beneath the epithelium. The duct is composed of a double 
layer of cylindric epithelium and fibrous tissue, intermingled with elastic 
fibers. The main duct divides and passes into the intralobular tubes, 
beyond which are the intermediate tubules. The intralobular tubes are 
lined by columnar cells, while the intermediate tubules are lined by 
elongated, spindle-shaped cells. The salivary cells lining the acini are 
different in character from those in the submaxillary and sublingual 
glands. The parotid gland is a true salivary gland, and the serous 
gland-cells composing its epithelial lining are disposed in a single layer. 
The cells are columnar or pyramidal in form and composed of a dense 
protoplasm, containing a spheric nucleus. 

The Submaxillary Gland. — The excretory duct (Wharton's duct), 
like the main duct of the parotid gland, is composed of a double layer 



NERVE-SUPPLY TO THE SALIVARY GLANDS. 345 

of columnar epithelium, external to which is a layer of cellular connective 
tissue, the whole being surrounded by a thin stratum of muscular fibers 
placed longitudinally. The intralobular tubes are lined by a specialized, 
elongated, cylindric epithelium, which, in the intermediate tubules, be- 
comes clothed with cubic cells. The acini are lined either with serous 
gland-cells similar to those lining the acini of the parotid gland or with 
mucous gland-cells, the former being most constant in their presence. The 
two kinds of acini are uninterrupted!/ connected. In most instances 
there are but a few mucous acini present within the lobule, but occa- 
sionally they are found in abundance. The submaxillary is a mixed 
or mucosalivary gland. 

The Sublingual Gland. — The excretory duct (Rivini's duct) is 
similar in structure to the chief excretory duct of the submaxillary gland. 
The intralobular tubes are lined with columnar epithelium. The inter- 
mediate tubules are not positively known to exist, and it is quite probable 
that the intralobular tubes pass directly into the terminal compartments. 
The acini are composed of a membrana propria and gland-cells, both 
mucous and serous. The former are much more numerous than in the 
acini of the submaxillary. The membrana propria is composed of 
stellate connective-tissue cells. This gland is also a mixed or muco- 
salivary gland. 

Blood-vessels and Lymphatics in the Salivary Glands. — The 
lobules of the salivary glands are richly supplied with blood-vessels. The 
many arterial branches break up into numerous capillaries, which, form- 
ing a dense network, surround the acini, being supported by the inter- 
alveolar connective tissue. The lymphatic vessels accompanying the 
intralobular tubes are in communication with numerous lymph-spaces 
which exist between the interalveolar connective tissue and the walls of 
the acini. The substance of the gland is further supplied with blood 
by numerous plexuses of lymphatics which are carried or supported by 
the interlobular connective tissue. 

Nerve-supply to the Salivary Glands. — The nerve-fibers distrib- 
uted to the salivary glands are both of the medullated and non-medul- 
lated variety, and other nerve-tissue in the form of ganglion-cells is 
present. The medullated nerve-fibers are abundantly numerous, and 
are distributed to all parts of the gland. In many respects the fibers are 
peculiarly constructed. They are extremely delicate, made so by the 
frail nature of this medullary sheath; they divide and give off so many 
branches as to almost give them a feathery fineness. This peculiarity 
is especially noticeable toward their extremities, where the fibers lie 



346 HISTOLOGY. 

between the alveoli and give off minute branches in all directions. The 
nerve-fibers are placed in close relation to the tubes and tubules, which 
they freely encircle; they perforate the membrana propria and breakup 
into finer subdivisions, from which they are distributed to the exterior 
of the epithelial cells. In the alveoli two kinds of nerve terminations are 
found. The primitive fibers branch between the alveoli and are distrib- 
uted to the membrana propria, upon entering which numerous branches 
are thrown off which pass to the epithelial cells beneath. The non- 
medullated fibers, which are much less numerous, are composed of an 
extremely delicate fasciculi of transparent fibers resembling axis-cylinders, 
and invested by a sheath of connective-tissue cells containing nuclei. 
The distribution of these fibers is similar to the medullated fibers, encircling 
the tubes and penetrating the membrana propria, being similarly distrib- 
uted to the alveoli. 

Literature. 

Stohr, "Text-book of Histology." 

Strieker, "Human and Comparative Histology," vol. i, 1870. 

Klein, "Elements of Histology," 1889. 

Sebastian, "Recherches anatomique, physiologiques, pathologiques, les Glans 

Labiales," 1842. 
Ward, "On Salivary Glands," Tood's "Cyclopedia of Anatomy and Physiology." 
Klein and Vernon, Strieker's "Human and Comparative Anatomy," vol. i, chap. 

\vi. 
Sudduth, "Embryology and Dental Histology," "American System of Dentistry," 

vol. i, part iii. 
Brubaker, "Transactions Odontological Society of Pennsylvania," i88o-'a5. 
Todd and Bowman, "Physiological Anatomy," vol. i. 
Szontagh, " Essays on Minute Anatomy of Hard Palate in Man," 1866. 



CHAPTER V. 

Muscular Tissues of the Mouth ; of the Lips ; of the Cheeks ; of 
the Soft Palate; of the Tongue. 

MUSCULAR TISSUES OF THE MOUTH. 

Muscular Tissues of the Lips. — The minute bundles forming the 
fasciculi of the oral sphincter muscle — the orbicularis oris — are distributed 
between the submucosa of the mucomembranous portion and the sub- 
cutaneous tissue of the cutaneous portion of the lips. The muscular 
fibers radiate in three principal directions upon either side of the median 
line: from the angle of the mouth toward the median line, and from the 
fleshy slips of the maxilla and mandible — the musculi incisivi. As the 
fibers from the angle of the mouth pass to the substance of the lip, they 
are arranged in a laminated manner. ' When the median line is reached, 
one set of fibers terminates somewhat abruptly in the subcutaneous tissue, 
another set is continued beyond the median line and attached to the cutis 
of the opposite side, while a third set, without crossing the median line, 
is attached to the incisive fossa? of the maxilla and mandible. The numer- 
ous muscular fibers of the internal labial or mucomembranous portion, 
and the external, facial, or cuticular portion, penetrate the parts and 
terminate in close proximity to the epithelium or to the base of the papillae. 
Delicate, hair-like fibers which are continuous with the sarcolemma 
slightly penetrate the cutis and membrana propria. A few of the fibers, 
which may be classed with the terminals of the outrunning muscles 
from the lips, are arranged in a number of fasciculi in the subcutaneous 
portion, pass through the fasciculi of the orbicularis oris, reach the sub- 
mucous tissue, where they cross and recross one another, and finally pass 
into the membrana propria, where they end in fan-like terminals. The 
fasciculi of the orbicularis differ somewhat in the upper and lower lips; 
in the former the bundles are strongly developed toward the angle of 
the mouth, while in the latter the median bundles are the strongest. 
The labial muscular tissues are of the transversely striated variety. The 
fibers are cylindric in form, having rounded or pointed extremities in 
the interior, and broad or flattened ends where they come in contact 
with the periosteum. When examined with a high power each fiber 

347 



348 HISTOLOGY. 

shows alternately broad and narrow striae, the former being dim, while 
the latter is bright in appearance. With a stronger power both the broad 
and narrow striae are seen to be transversely striated. 

Muscular Tissues of the Cheeks. — The muscles entering into the 
construction of the lateral walls of the mouth have already been described 
in part I, page 12, giving the relations existing between the individual 
muscles, together with the general disposition of the various fasciculi. 
Histologically considered, these muscles partake of all the characteristics 
of striated or voluntary muscular tissue. In the body of the buccinator 
and masseter muscles the fibers are cylindric and have definitely pointed 
or rounded ends. Near their termini, particularly in the latter muscle, 
the inner extremities of the terminal fibers are pointed, while the outer 
ends, or those by which the attachment is formed, are broad and rather 
flat. 

Muscular Tissues of the Soft Palate. — The disposition of the 
striated muscular tissue of the soft palate is extremely complicated. The 
azygos uvula, the only true longitudinal muscle in the soft palate, has 
its origin from aponeurosis of the soft palate and from the nasal spine of 
the palate-bone, the fibers passing backward upon either side of the 
median line. This is a double muscle, and near its point of origin the 
two portions are distinct and separated by a definite space, but upon 
reaching the base of the uvula they become closely associated. The 
fasciculi do not continue to the apex of the uvula, but immediately beyond 
the center of its length are thrown out fan-like toward the sides, terminat- 
ing in a manner similar to the fibers of the lips. In passing from before 
backward a number of small fasciculi are given off, which reach out 
laterally and traverse the glandular lobes, completely surrounding them, 
after which they again return to the principal fibers at the median line. 
The palatopharyngeus muscle is divisible into two parts, the upper 
extremities of which lie partly in front and partly behind the levator 
muscles. The greater number of the fibers of one set, situated in front 
of the levators, form a curved, flattened aponeurosis. The fibrous 
border of the hard palate serves as an attachment for the convex border 
of this portion, while the other border, which is concave, is directed 
toward the arch of the levators. The fibers of the palatopharyngeus, 
situated behind the levators, form a number of loose fasciculi inter- 
spersed by fat-cells. In passing toward the free border of the soft palate 
the fibers become much more delicate, and, separating, some course in 
front and others behind this muscle. In this location the fibers become 
closely associated with the glands, and either end here or are continued 



MUSCULAR TISSUES OF THE TONGUE. 349 

to the submucosa, or even to the membrana propria of the mucous 
membrane. The fibers of the palatopharyngeus unite with the fibers of 
the levators, and an arch-like fasciculus is formed by this union which, 
subdividing, passes in front of the azygos uvulae to the opposite side. 
All of these fibers run outward and downward, and unite with the 
extremities of the other palatal muscles, the fibers of which are some- 
what more regularly distributed. Like the muscles of the lips and 
cheeks, the several fasciculi of the palatal muscles form a delicate 
plexus, and a quantity of fatty tissue is found between the various 
fasciculi. 

Muscular Tissues of the Tongue. — The tongue is divided into 
two equal lateral portions by a median septum — the septum linguce. 
This central septum, composed of a vertical layer of compact, fibrous, 
connective tissue, extends the entire length and depth of the lingual 
median line. Beginning at the hyoid bone, it gradually increases in 
prominence until the middle of the organ is reached, beyond which point 
it becomes less pronounced and finally disappears near the tip. The 
bundles of the muscular tissues are arranged longitudinally, transversely, 
and vertically. The former lie immediately beneath the mucous mem- 
brane, including the superior lingualis above and the inferior lingualis 
below, together with the greater part of the styloglossus. The superior 
lingualis extends from the base to the tip of the organ, and by short 
fasciculi its fibers are attached to the overlying tissues. The fibers of 
this muscle are placed between the hyo- and styloglossi muscles of the 
opposite side, both of which overlap the fibers of the lingualis near the 
base of the tongue. The inferior lingualis also gives off several small 
fasciculi and fibers to the mucous membrane beneath, and is composed 
of two bands which reach from the base to the apex, each being 
placed between the hyoglossus and genio-hyoglossus muscles. The 
transverse fibers, which are placed between the superior and inferior 
lingualis muscles, originate from the septum linguae, and form the bulk 
of the organ. From their point of origin these fibers course outward and 
upward to the sides of the tongue. Those fibers which are vertically 
disposed decussate with the transverse fibers, and pass from the dorsum 
toward the under surface of the tongue, the fibers curving gracefully with 
their concavity directed toward the under surface. In most instances 
the ascending vertical fibers, as well as the transverse fasciculi, pass 
between those longitudinally disposed and connect with the submucosa. 



CHAPTER VI. 

Tissues of the Teeth — Enamel; Dentin; Cementum; the Tooth- 
Pulp. — The Alveolodental Membrane. 

TISSUES OF THE TEETH. 

Enamel. — The enamel, which forms a cap-like covering of varying 
thickness over the entire crown of the tooth, is a vitreous, hvaline sub- 




FlG. 2J5. — Section of Enamel from Human Tooth (Specimen by J Howard Mummery.) 

X 350. {After Williams.) 

stance, containing but little, if any, organic matter. The thickness of 
the enamel cap appears to be strongly influenced by the function of the 

35° 



ENAMEL. 351 

different parts of the tooth-crown, being thickest over the cutting-edges 
of the anterior teeth, while in the cuspidate teeth, the entire occlusal 
surface is provided with the thickest enamel layer. It is about evenly 
distributed over the lateral walls of the crown, but as the cervical line is 
approached, its thickness is gradually diminished (Fig. 226). Chemically, 
enamel is composed of the salts of lime, calcium phosphate predominat- 
ing. Calcium carbonate, magnesium phosphate, and calcium fiuorid 
are present in smaller quantities. The proportionate quantity of lime- 
salts in enamel is not fixed, a slight variation in density occurring in the 
enamel of different individuals. These essential differences are regulated 
by the proportionate quantity of calcium phosphate and carbonate — a 
greater amount of the former being productive of additional hardness, 
while an increase in the latter beyond the minimum amount decreases 
this quality. As a general rule, the teeth of males contain a greater 
amount of calcium phosphate than the teeth of females, as shown by the 
following analysis by von Bibra: 



Calcium phosphate and fluorid, 89 

Calcium carbonate, 4 

Magnesium phosphate, 1 

Other salts, 

Cartilage, 3 

Fat 



AN. 


Woman. 


.82 


81.63 


•37 


8.88 


•34 


3-55 


.88 


•97 


■39 


5 97 


.20 


a trace 


■59 


5-97 



Total organic, 3 . 

Total inorganic, 96.41 94 .03 

In general structure enamel is composed of numerous hexagonal 
prisms, with a common direction at right angles to the long axis of the 
tooth. These prisms are known as enamel prisms, enamel fibers, or 
enamel rods. While the general direction of the fibers is, as previously 
stated, nearly at right angles to the body of the tooth, they do not pursue 
a perfectly straight course in passing from the dentin to the surface, but 
are disposed in a tortuous or wave-like manner. The enamel prisms 
may be said to sit on end against the surface of the dentin, minute depres- 
sions in the latter receiving the extremities of the rods. The direction 
of the enamel prisms, as compared to the body of the tooth-crown, varies 
according to the part of the crown which they occupy. Taking the 
entire crown of the tooth, they radiate in such a manner from the surface 
of the dentin that at the cutting-edge or occlusal surface of the tooth they 
are more or less vertical, while over the lateral surfaces they tend to the 
horizontal direction. An examination of the prisms whemisolated and 



352 HISTOLOGY. 

decalcified exhibits numerous evenly distributed varicosities, producing 
a transversely striated appearance to the rods. Tomes has pointed out 
that the enamel rods are variously disposed in that portion of the enamel 
most closely associated with the dentin. On the cusps of the teeth they 
are twisted and curved in various directions, while near the surface on the 
incisors they are uniform and straight. In general, the enamel rods, 
which begin on the surface of the dentin, are continuous through the 
entire thickness of the enamel. In passing from the interior to the exte- 
rior, the individual rod, to occupy a proportionate space in all parts, 



Enamel 




Fig. 266. — Enamel and Dentin from Human Tooth, showing Gradual Reduction in the 
Thickness of the Former as the Cervix is Approached. X 20. 

would have to increase in diameter; but this it does not do. In conse- 
quence of this arrangement there exist numerous supplemental or per- 
ipheral rods, which extend but a short distance from the surface, filling in 
the interprismatic spaces formed by the longer rods. With the exception 
of the faint transverse striations, the enamel prisms appear to be structure- 
less. A variety of opinions have been expressed in regard to the cause 
for the striated appearance of the enamel rods. It was claimed by Hertz 
to be attributable to a temporary arrest of calcification, but more recent 
investigation has shown the cause to be the presence of varicosities in the 
individual fibers, precisely as the varicosities in the muscular fibers pro- 



ENAMEL. 



353 



duce the striated appearance in that tissue. Bodecker asserts that fully 
developed normal enamel is non-striated, and Von Ebner practically 
makes the same statement, claiming that they are due to the preparation 
of the specimen, which usually being mounted in Canada balsam, suffers 
sufficiently from the slight acid reaction to produce the striated appear- 
ance. These statements Williams emphatically denies, saying that, while 
in some specimens the varicosities are apparent in some parts, they are 
decided in others. If this be accepted, it would seem to entirely over- 
throw the theory of Von Ebner in regard to the action of the acid, which 
would be distributed to all parts alike. 




Dentin. 



Enamel. 



Fig 267. — Comparison in the Appearance of the Enamel and Dentin under Low Power 

of the Microscope. X 40. 

^According to Williams, the varicosities of one enamel prism are 
opposite those of the adjoining prisms, and by the coming together of 
the varicosities the prisms become united by means of processes which 
they send out. In like manner the varicosities upon the same rod are 
connected by processes running parallel with the prism. According to 
Von Ebner, enamel is traversed by numerous minute canals, and Heitz- 
mann claims to have found organic fibers in its substance. Williams, 
while admitting the enamel structure to be far more complex than past 
research has shown, appears to have fully demonstrated that neither canals 
nor organic fibers are present — in fact, he denies the presence of the least 
23 



354 



HISTOLOGY. 



trace of organic matter in this structure. The interprismatic matrix, 
heretofore considered by most authorities to be an organic structure, 
now appears, by the thorough methods employed by the last-named 
gentleman, as a transparent, inorganic substance. By numerous experi- 
ments he was enabled to secure a specimen in which the interprismatic 
spaces of one layer were not backed up by the rods of another layer. 
In some instances the specimen showed the rods well separated and the 
interspace closed by a perfectly transparent substance, in the interior of 
which might be seen connecting processes passing from one rod to another. 



Enamel 
Rods Fully 
in Trans- 
verse Sec- 
tion 



Enamel 
Rods vari- 
ously Dis- 
tributed 




Enamel 
Rods Not 
Fully in 
Trans- 
verse 
Section 



Enamel 
Rods of 
Irregular 
Form 



Fig. 268. — Human Enamel Transverse Ground Sections. (After Cy. si.) 



That the enamel is practically of inorganic material, and therefore not 
capable of transmitting or receiving sensations, may be demonstrated by 
the simple experiment of immersing a thin section of this tissue in a weak 
solution of chromic acid, the result of which will be a speedy separation 
of the enamel prisms which have been liberated by the destruction of the 
interprismatic substance. What does this signify? Chromic acid is 
one of the best preservatives of organic tissue known, and if the cement- 
ing substances in enamel were organic or even partly so, the prisms by 
this test would not be freed, but instead would become more firmly ce- 
mented together. Therefore we infer from this that the interprismatic 



ENAMEL. 355 

substance is even less highly organic than the prisms themselves, these 
not being acted upon until after the material which holds them together. 
In addition to the striated appearance formed by the varicosities of the 
individual prisms of fully developed enamel, other structures of a different 
character, and upon a much larger scale, are present and known as the 
"brown stria oj Retzius" (Fig. 269). These markings, readily seen 
with a low power, are of a brownish color, and run nearly parallel with 
the surface of the dentin or enamel. Those striae nearest the surface of 
the dentin are inclined to follow the contour of that structure, extending 




Fig. 269. — Thick Section of Enamel of Human Tooth, showing Brown Striae of Retzius. 

X 40. 

in many instances the entire length of the crown. The lines nearest the 
surface are longest in the region of the cutting-edge, or occlusal surface 
of the crown, becoming shorter as the neck of the tooth is approached, 
being directed at an acute angle to the surface of the dentin at that point. 
A number of theories are advanced to account for the presence of the 
"brown striae of Retzius." Tomes suggests that, coinciding as they do 
with the outer surface of what was at one time the primitive enamel cap, 
they might be considered as in a measure outlining the stratifications of 
the primary deposit. Another theory, but one seemingly without founda- 
tion, is to the effect that the striae are produced by an arrest in the calci- 



356 



HISTOLOGY. 





— W— ■—&&? :Vf '"J3r3 


iLl^ 


3rown striae 
of Retzius. 

Lines of f 
Schreger. ( 


w 


■ — '** i£&-* - K J -v? J 



Enamel. 



Dentin. 



Fig. 270. — Enamel and Dentin, Human Tooth. (After Gysi ) 




Oral Epithelium 
Heaped up 
over Band 



Band 



Connective Tissue 



FlG. 271. — Vertical Section through Jaw of Human Embryo. Formation of Tooth-band, 

about Sixtieth Day. X 300. 



ENAMEL. 



357 



fying process; while a third theory attributes the cause to a variation in 
the character of the nourishment taken by the mother during pregnancy. 
The acceptance of this latter theory would seem to indicate a set diet 
for all mothers and at stated intervals, the striae always being present 
and somewhat regularly distributed throughout the tissue. 

Still another set of lines or markings are to be observed in the sub- 
stance of sections of enamel, these being known as the "lines of Schreger." 
Figure 270 shows these lines as they appear in the enamel by reflected 
light, the same being quite invisible by transmitted light. The presence 

Epithelium 






.'.;.- (■■■ 



Band 







(Older 1 

I Layer 
Infant 
Layer 



Connec- 
tive 
Tissue 



Fig. 272. — Vertical Section, Tooth-band, Human Embryo. Tenth Week. X 300. 



of these lines is due to the various directions assumed by the contiguous 
groups of enamel rods. Beginning at the surface of the dentin they are well 
defined, but gradually become less marked as the exterior of the enamel 
is approached. At the line of union between the enamel and dentin ir- 
regularly formed cavities are occasionally observed, into which the denti- 
nal tubules may extend, and in rare instances individual tubules may 
pass beyond the boundary-line of the dentin and enter the enamel, but in 
all probability both of these conditions are pathologic. Such a state of 
affairs could hardly be considered normal when we take into considera- 
tion that the dentin and enamel calcify in opposite directions, and that the 
outer wall of the former is completed before enamel calcification begins. 



358 HISTOLOGY. 

Development of Enamel. — Preparations for the development of the 
enamel begin toward the close of the second fetal month, appearing first 
as a multiplication of the primitive epithelial cells in the form of a con- 
tinuous linear projection extending somewhat obliquely into the sub- 
jacent connective tissue. From this crest, or tooth-band, the germs for 
the future enamel organs are given off. These primary dental bulbs, 
as they are called, number one for each tooth to be generated, and coinci- 
dently with their appearance an aggregation of closely associated connec- 
tive-tissue cells make their appearance in the surrounding submucous 

Dental Ridge 




$m A 



Band w-»**fcy33l 

Epithelium 



Location of Future 
Enamel Organ 



******&• 

.:>>.• 



Connective Tissue 



Fig. 273. — Same as Figure 272. About Twelfth Week. X 200. 

tissue. This papilla-like specialization of the submucous tissue is the 
primitive dentin germ, or dentin papilla. It will thus be observed that the 
enamel is a product of the surface epithelium, ectodermic, while the dentin 
is generated from the connective tissues, mesodermic. Soon after the ap- 
pearance of this club-shaped thickening, or tooth-bulb, by further differ- 
entiation its form becomes bell-shaped, with the concavity directed toward 
the surface. The dentin papilla gradually pushes into the concavity 
of the forming enamel organ, and at a later period the odontoblastic cells 
are generated about the periphery of the papilla, closely followed by a 
surface calcification of the dentin. Soon after the forming of the external 
layers of dentin the ameloblasts or enamel-forming cells become active, 



ENAMEL. 



359 



and a deposition of enamel prisms takes place upon the exterior of the 
dentin cap. 

Before taking up the subject of enamel calcification, brief reference 
will be made to the further development of the enamel organ. As the 
growth of this organ proceeds there is, as the result of a rapid proliferation 
of the cellular structure, a marked tendency for the organ to become 
separated from the tooth-band. The peripheral cells or external epithe- 
lium are columnar or prismatic, and remain so, while those in the center, 
primarily polygonal, soon become transformed into a radiating network 




Surface of Jaw 



Epithelium 
Upper Jav 



Epithelium X 



■> ,'i 




"• - ■ c? ' f.-y : 



Wg 



Neck of Cord 



■ Stellate Retic- 
g ulum of 

Enamel Organ 

I Enamel Organ 
I 

Dentin Papilla 



Forming Bone 



Fig. 274. — Vertical Section through Tooth-band, Cord, Dentin Papilla, and Enamel Organ, 

about Fifteenth Week. X 150. 

or stellate reticulum. The appearance of a stellate reticulum is first 
observed to take place in the cells occupying the central portion of the 
enamel-organ, this cellular transformation progressing from the center 
outward, but ceasing before reaching the columnar surface cells contigu- 
ous to the dentin papilla. Between the enamel cells and the stellate 
reticulum is a layer of unaltered cells — the stratum intermedium. In 
the earlier stages of the development of the enamel organ the peripheral 
cells are alike, being columnar or prismatic, but almost coincident with 
the appearance of the dentin papilla, the cells most closely related to it 
are observed to become elongated, and form the internal epithelium of the 
organ. As the cells forming this internal epithelium become elongated, 



3 6 ° 



HISTOLOGY. 



their nuclei, instead of occupying the center of the protoplasmic body, 
are carried to their extremities. It will thus be seen that the completed 
enamel organ consists of four divisions or layers of cells. Beginning 
with its convex surface is an external epithelium or outer tunic, succes- 
sively followed, in passing toward the dentin papilla, by a stellate reticu- 
lum, stratum intermedium, and an internal epithelium or inner tunic. 
As the growth of the enamel organ proceeds, the tooth-band becomes 




FlG. 275. — Section of a Developing Tooth of Lamb. 
a, Ameloblasts; D, dentin; P, dental pulp. 



X 600. 



smaller and smaller in size, until finally a complete rupture takes place. 
This rupture, however, does not occur until the enamel organ has about 
or fully completed its development, and, after remaining so long under 
the influence of the oral epithelium, it must be considered, as before stated, 
an epithelial structure. It is through the agency of the internal epithelial 
cells of the enamel organ, the enamel cells or ameloblasts, that calcification 
of the enamel take? place, and that subject will next be considered. 



ENAMEL. 



;6i 



Amelification (Fig. 275). — -Two theories are advanced in regard to 
the calcification of the enamel. In one it is claimed that the ameloblasts 
or enamel cells became directly calcified or converted into enamel; in the 
other the ameloblasts are simply considered as controlling agents, by 
secreting or depositing the calcium salts which form the enamel prisms. 
In the latter theory it is generally believed that the enamel is secreted or 
shed out from the extremities of the ameloblasts, thus being productive 




Dentin 



ffim 



Enamel 
I Rods 



mm 







ft Outer 






£— Layer of 
Enamel 




WMMkiW^ 






JlhI MrJt ' 


Amelo- 




uNC **9E<SKjE ** / 


blasts 

Section of 
Papillae 



Fig. 276. — Section of Incisor of Rat. X 200. (After Williams.) 



of enamel fibers corresponding in size and position to the secreting cells. 
By the direct calcification of the ameloblasts, it would be natural to 
expect the process to begin on the exterior of the cell and gradually pass 
into its interior, the central portion being the last to calcify. This 
would result in an enamel prism corresponding in size and form to the 
generating cell, and in a measure this similarity between the calcified 
and uncalcified structure does exist and is one of the potent factors 



362 HISTOLOGY. 

in the recognition of this theory, but it is hardly sufficiently convincing 
to warrant a general acceptance of the belief. 

By examining figure 275 the importance of the secretory theory is 
favored. Here we note that the early formed enamel at A records what 
appears to be the definite action of the ameloblasts by prolongations of 
partly calcified tissue extending from the cells, these markings corre- 
sponding in number and location to the cells themselves. Between these 
prolongations and the ameloblasts are many highly refractive granular 
bodies which seem to be the actual lime deposit shed out from the free 
extremities of the cells, from which they pass into the substance of the 
then organic matrix beyond at A and form the enamel prisms. Williams 
takes exception to this latter view, substantiating his opinion by stating 
that while the ameloblasts of many animals are similar in shape and 
arrangement, the enamel produced from these similarly arranged cells 
varies greatly in structure. The same writer also states that, when such 
a similarity of arrangement exists between the ameloblasts and the enamel 
prisms, it occurs near the commencement of enamel calcification, and 
that at a later period the relative position of the ameloblasts and the 
prisms is always in longitudinal section. Dr. Williams calls attention 
to the fact of the enamel prisms or rods not extending through the entire 
distance between the enamel cells and the calcified dentin (Fig. 275). 
That part of the structure lying between the ameloblasts and the extrem- 
ities of the enamel rods is made up of a double set of fibers, some of 
which are almost at right angles with long axis of the ameloblasts. In 
figure 277 D the two sets of fibers previously mentioned are found to 
join and become closely interwoven. 

The ameloblasts are connected with the cells of the stratum inter- 
medium, and more recent investigation goes to prove that this latter 
structure is directly interested in furnishing to the ameloblasts the proper 
material for the calcifying process. The stratum intermedium cannot, 
however, take part in the primary enamel calcification, as this process 
commences before the stratum intermedium is fully developed. This 
being the case, it is generally supposed that the stellate reticulum furnishes 
the material for the upbuilding of the first enamel prisms. 

Lying between the free extremities of the ameloblasts and the enamel 
in the course of formation is what has been generally considered as a 
structureless basement membrane or membrane praformativa. The 
existence, exact location and structure of this membrane has been and 
still remains a matter of conjecture. The generally accepted theory 
appears to be that given above, but Williams refers to it as a layer of 



ENAMEL. 



;63 



newly formed enamel, and does not consider it as a structureless mem- 
brane. The structure is to be observed in figure 277, at either extremity 
of the ameloblastic cells, this writer claiming that the so-called structure- 
less membrane is present at both of these points. 

As to the formation of the enamel rods, Dr. Andrews, in 1894, 
referring to the presence of calcoglobulin in the enamel cells considered 




Outer 
Fibrous 
Enamel 

Layer 



Ameloblasts 



Position of Capillary Loop 



Fig. 



277. 



-Section of Incisor of Rat, showing Partial Decalcification of Enamel. 

{After Williams.) 



X 600. 



these refractive bodies as calcospherites, which, after being taken up by 
the ameloblasts, were excreted by them, and after coalescing formed 
globules of larger size, from which the rods were built up. Williams 
partly agrees with this statement, but he is of the opinion that the calco- 
spherites coalesce while in the ameloblasts, forming large, spheric bodies, 
but that the deposit of this substance is in no way productive of building 



3 6 4 



HISTOLOGY. 



the enamel rods. The theory of Tomes in regard to the forming of 
enamel rods was that the walls of the ameloblasts themselves became 
calcified, while the contents of the cells also became solidified, the first 
forming the interprismatic substance, while the second became the 
enamel rod. Whatever theory be accepted as to the formation of the 




Fig. 278. — Section of Developing Tooth of Embryo Lamb. X 150. (After Williams.) 

a, Forming Dentin; 6, Forming Enamel; c, Stratum Intermedium; d, Inner Ameloblastic 
Membrane; e, Outer Ameloblastic Membrane; /, Ameloblasts. 

enamel rods or prisms, there appears to be no question in regard to the 
general process of enamel calcification. In the first place, an organic 
matrix is formed, into which the first-formed layer of enamel is deposited. 
Gradually the organic matter disappears, leaving behind the inorganic 
elements closely resembling in appearance the organic matrix, which 
it has by atomic change supplanted. The question of an organic inter- 



ENAMEL. 



365 



prismatic cement-substance is also one upon which various writers dis- 
agree. Klein partly believes that such a substance does exist, basing 
his opinion upon the fact that the ameloblastic cells, in common with all 
epithelial cells, are separated from one another by a homogeneous inter- 
cellular substance, and that a certain proportion of this organic substance 
must remain between the enamel prisms after calcification. Dr. Sudduth, 
by a series of experiments made some years ago, appeared at that time 




Fig. 279. 



to have furnished conclusive proof that an organic, interprismatic cement- 
substance does not exist between the enamel prisms of fully developed 
enamel. By the use of a dilute solution of chromic acid, the action 
of which is the preservation of organic substance, the prisms were liberated, 
which would not have been the result had they been cemented by an 
organic cement-substance. By substituting dilute muriatic acid, the 
action of which is the destruction of organized tissue, the prisms were not 



366 HISTOLOGY. 

liberated, the acid acting evenly upon the whole mass of enamel, and 
finally resulting in its complete destruction, not leaving the slightest trace 
of an organic matrix behind. Dr. Williams claims that this transparent 
cement-substance is formed by the distribution of a translucent liquid 
substance about the previously formed pattern for the enamel rods. 
This pattern, generated through the activity of the enamel cells, is com- 
posed of a translucent material somewhat more solid than that substance 
which surrounds it. These two substances calcify together, the latter 
forming the enamel prisms, while the former creates the cement or inter- 
prismatic substance. There is no better method by which to study the 
character or mode of development of a growing or matured tissue than 
by an artificial disassociation of its component parts. In enamel it 
matters but little in what part of the tissue or at what period of its growth 
the examination be made to learn of the action of the decalcifying agent; 
it will never be found to take place in a manner corresponding to the 
direction of the enamel fibers, but decalcification takes place in older 
enamel more in the form of a general breaking up of the structure, as 
shown at A in figure 279, while at B, which represents the newly formed 
tissue, the action is one which appears to indicate a breaking up of the 
interprismatic substance, favoring the theory of secretory amelification. 

This distinction between the enamel cells and their product possesses 
none of the characteristics to properly classify it as structureless. There 
is but little doubt as to its character, being the primary product of the 
ameloblasts, while the corresponding zone at the distal end of the 
enamel-forming cells results from the functional activity of the stratum 
intermedium. 

Figure 288 shows a section near the point of the cusp of a developing 
molar and exhibits a portion of the enamel organ at a time immediately 
prior to the beginning of enamel calcification. A is the uncalcified dentin, 
B the ameloblasts, now closely associated, and a very regular layer of 
elongated cells, and behind these another layer of cells, which undoubt- 
edly serve as feeders to the ameloblasts, the stratum intermedium. This 
section is especially valuable in that it shows a number of capillaries 
distributed through the body of the stellate reticulum and actually pene- 
trating the stratum intermedium, as seen at B. 

When viewed with a low power, these minute blood-vessels appear 
to form a complete network, and in the district between the cusps pervade 
the entire structure, from the stratum intermedium on one side to the 
same cells on the other. The appearance of this animated vascular sup- 
ply to the enamel is coincident with the process of calcification, for during 



ENAMEL. 



367 



the early life of the tooth-germ it is non-vascular. The growth of enamel, 
strata upon strata, from within outward is therefore by the direct cal- 
cification of the enamel cells or ameloblasts, and while this is going on, and 
as long as the crown of the tooth is incased in its epithelial cap, the enamel 
organ, the growth of the tissue is stimulated through the blood-vessels 




Fig. 280. 



everywhere present in the stellate reticulum. The presence of this special- 
ized blood supply to the central portion of the enamel organ was for a long 
time doubted, but at present it can be readily observed (Fig. 279). As 
soon, however, as the tooth passes through the surface tissue, carrying 
with it the external epithelium of the enamel organ as the enamel cuticle, 
the possibility of nourishment has been cut off, and after a little time it 



368 HISTOLOGY. 

becomes a petrified dental epithelium, no longer nourished and absolutely 
non-vital. 

Dentin.— This tissue, which constitutes the principal bulk of the 
hard part of the tooth, forms a complete cap-like investment over the 
pulp, from which it is generated. It is white or slightly yellowish-white 
in color, somewhat elastic, and a trifle harder than bone, which it resem- 
bles in many of its characteristics. In a perfectly developed tooth no part 
of the dentin appears upon the surface, that part within the crown being 
covered by the enamel, while that of the root is inclosed by the cementum. 



■ Dentin 
Enamel 




Fig. 281. — Section through Crown of Human Cuspid. X 30. 

While the thickness of the dentin varies somewhat over the different parts 
of the tooth, there is a decided disposition to an equal distribution in 
every direction. Dentin, unlike enamel, consists of an organic matrix — 
a reticular tissue of fine fibrils richly impregnated with the salts of cal- 
cium, in this resembling the matrix of bone. Traversing the matrix 
are long, fine canals or tubes (Fig. 289) — the dentinal tubules — which 
pass from the margins of the pulp toward the surface. Immediately 
surrounding the dentinal tubules the matrix is especially dense, forming 
a lining or sheath to the tubes, known as the dentinal sheaths. Occupying 
the lumen of the dentinal tubules are solid elastic fibers — the dentinal 
fibers. Dentin, therefore, presents for examination, first, the matrix; 



DENTIN. 369 

second, the dentinal tubules; third, the dentinal sheaths; and fourth, 
the dentinal fibers. 

The Matrix. — As previously stated, the matrix is composed of 
organic and inorganic substances, but the proportionate quantity of 
organic and inorganic constituents is so variable that it is impossible 
to furnish a definite chemic analysis. The relative quantity of organic 
and inorganic matter is not only variable in the teeth of different individ- 
uals, but is continually changing in the teeth of the same individual, the 
former being present in larger quantities during youth and gradually 







Fig. 282. — Section through Root of Human Incisor, showing many Dentin Tubules in 

Transverse Section. X 200. 

diminishing as age advances. From an examination of perfectly dried 
dentin, the following approximate analysis has been obtained: 

Organic matter (tooth-cartilage), 27.61 

Fat, 0.40 

Calcium phosphate and fluorid, 66.72 

Calcium carbonate, 3-36 

Magnesium phosphate, 1.08 

Other salts, 0.83 

The organic basis of the matrix appears to be structureless and 
transparent, and, although closely resembling the matrix of bone, is 
not identical with it. While the matrix is usually structureless, there 
24 



37° 



HISTOLOGY. 



are instances in which the presence at one time of connective-tissue fibers 
is indicated. 

The Dentinal Tubules (Figs. 283, 284, 285). — Beginning by a free 
opening about the walls of the pulp-cavity, the dentinal tubules permeate 
the matrix in all directions. The tubules are generally disposed in a 
direction, perpendicular to the surface, so that in different parts of the 
tooth they radiate in various directions. Beginning upon the surface 
of the pulp-cavity, at which point they are of greatest diameter, they 
pass more or less in a spiral manner toward the surface (Fig. 285), before 
reaching which they become gradually reduced in size, as a result of the 



Lacuna . ■*. _"--'V 

3k 



Dentinal Tubules 




£ Cementum 



Dentin 



Fig. 283. — Dentin and Cementum from Root of Human Molar. (After Gysi.) 

numerous branches which they give off (Fig. 284). The branches given 
off from the main tubes are quite variable in size, and anastomose with 
one another or with the branches from other tubules. In the region of 
the pulp the tubules are so closely associated that but little space is pro- 
vided for the intertubular substance or matrix; but as the surface is 
approached they become more widely separated, and, in consequence, 
the matrix substance is present in greater abundance. While the general 
direction of the tubes is perpendicular, they do not pursue a direct course, 
but are more or less curved as they pass from within outward. The 
curvature of the tubuli may be divided into two classes — long curves 
and short curves — usually referred to as the primary and secondary curva- 
tures of the dentinal tubules. The primary curvatures are few in number 
and are most prominent in the crown, while the secondary curvatures, prin- 



DENTIN. 371 

cipally found in the roots, are smaller and more numerous. The branches 
from the main tubes terminate in various ways, either by anastomosis, 
by gradually fading out into hair-like terminals, or by ending in hooks 
and loops. In rare instances they are said to enter the substance of 
the enamel or cementum, but it is doubtful if they do so normally. The 
branches from a main tube are usually two in number, the latter being 
almost equal in diameter to the former, and from this first set of branches 
a number of minute branches are given off almost at right angles. In 
the crown this latter class of tubules are seldom observed, excepting 
near the enamel margin, but in the root they are everywhere noticed. 




Fig. 284. — Longitudinal Section through Root of Human Molar. Branching of the 

Dentinal Tubules. X 200. 

Small varicosities are frequently present, but not in sufficient numbers 
to produce a striated appearance on the surface of the dentin. 

The Dentinal Sheaths. — While the dentinal tubules ramify through 
the matrix in the form of well-defined channels, the walls of the channels 
are not formed by the matrix, but by an indestructible substance the 
exact character of which is not fully understood. The walls of the tubes, 
or the dentinal sheaths, as they are termed, are believed by some histolo- 
gists to be calcified, while others, though acknowledging their apparent 
indestructibility, are doubtful as to the correctness of this theory. Neu- 
mann being the first to accurately describe the walls of the tubules, they 
have become known as "Neumann's sheaths." The existence of the 



372 



HISTOLOGY. 



dentinal sheaths may best be demonstrated by subjecting the tissue to 
the action of strong acid for a sufficient time to destroy the intervening 
matrix, which process usually requires several days. The fibrous mass 
remaining will be found to contain a collection of tubes, which, however, by 
careful examination, are found not to be the dentinal tubules themselves, 
but the walls of these canals. Magitot and Sudduth deny the existence of a 
wall to the dentin tubes. Tomes, while inclined to the belief that the tubes 
are provided with definite walls, suggests that they may have been pro- 
duced artificially during the preparation of the specimen, and that they 
are only brought into existence by the action of the agent used for this 




Fig. 285. — Transverse Section through Root of Human Molar, showing the Curvature of 
the Dentin Tubules about the Pulp-canal. X 40. 

purpose. In conclusion, the same writer adds that that part of the 
matrix immediately surrounding the fibril differs in its chemic con- 
stituents from the body of the matrix. 

The Dentinal Fibers. — Occupying the lumen of each dentin tube is 
a soft, elastic fiber, which is continuous with and has its origin from the 
odontoblastic cells upon the periphery of the pulp. The existence of 
these elongated processes of the odontoblasts having first been demon- 
strated by Tomes, they are otherwise known as Tomes' fibers. By 
means of these fibers, which not only fill the lumen of the larger tubes, 
but the minute branches as well, the substance of the dentin is both 



DENTIN. 373 

nourished and rendered slightly sensitive. There is still some doubt as 
to the real nature of the fibrils, but, if they are processes from the odonto- 
blasts, it would appear that the substance would be identical with that 
of the cell-protoplasm. Bodecker claims that they are not round but 
inclined to angularity, but Tomes infers that this form has been produced 
by the action of some reagent. Klein advances the theory that the odon- 
toblasts are active in the generation of the matrix for the dentin only, 
and that the dentinal fibrils are not processes from them, but originate 
from cells intervening between the odontoblasts and connecting with the 
dentin tubes. It has never been fully demonstrated that true nerve- 




'"'' 



Fig. 286. — Longitudinal Section through Root of Human Tooth, showing Primary 
Curvature of Dentin Tubules. X 40. 

fibers enter the dentin along with or in the substance of the dentinal 
fibril, but, while the evidence is not at present forthcoming, there is but 
little doubt that the sensitiveness of the dentin is produced by the presence 
of organized tissue in the tubuli. Some contend that the contents of the 
tubules are made up of, first, a creative portion, that given off directly from 
the odontoblasts; second, a circulatory portion, a minute vessel travers- 
ing each tubule, entering either by the side of the cell-bodies or passing 
through them, and that the nerve terminals are distributed in the same 
manner. Others say that minute nerve-filaments from the pulp pass 
directly through the odontoblasts and are continued in the center of the 
tubule surrounded by a simple connective tissue, the cell process, and 



374 HISTOLOGY. 

that in this way sensations are conveyed. It is now generally conceded 
that dentin is a highly organized connective tissue; that it has a circula- 
tory system and is endowed with sensation to a slight degree; that these 
conditions are brought about not by actual entrance into the tubules of 
separate vessels and nerve-filaments, but more in the way of the tubules 
being occupied by a general connective-tissue substance resembling in 
all essential features the pulp itself, being the semi-fluid interfibrillar 
ground-substance of the pulp; that dendrites of sensory neurons every- 
where present in the pulp, after losing their medullary sheaths divide 
into fine varicose fibers and become closely associated with the peripheral 



Cemen- 

tum 



1 JWM, 



Dent 















I 



m 

)f . Pulp-canal 




Fig. 287. — Transverse Section through the Root of a Human Incisor, showing the Dentin 
Surrounded by the Cementum. X 30. 

cells, pass between these, and enter the cone-shaped openings of the 
tubules and terminate soon after doing so. 

While the microscope reveals in some instances what appear to be 
prolongations from the dentinal fibers penetrating the enamel, or between 
its prisms, such a condition is improbable if not impossible. If this 
arrangement is present at all, it is so slight as to have no influence what- 
ever over the enamel either as to nourishment or sensation. No conclu- 
sions can be drawn with positive certainty from sections, since the slightest 
deviation from parallelism in the surfaces may easily produce deceptive 
appearances. It is just as common, and even more so, to find hair-like 
lines interwoven and running parallel with the surface of the dentin 



DENTIN. 



375 



immediately between this tissue and the enamel, as it is to see slight 
fibers crossing beyond their boundary-line to penetrate the enamel. 
The most likely place of all to find such a condition would be in the 
beginning of calcification, and here it is never observed. The peripheral 
pulp-cells, usually all classed as odontoblasts, are never found outside 
their own territory, the dentinal papilla; but their location in the beginning 
on the very surface of the papilla, almost in direct contact with the inner 
tunic of the enamel organ, would make it possible for their processes, 
when appearing, to penetrate between the cells of the enamel organ if 
they were grown out from the body of the cells from which they spring. 






Ceraentum \ 




Dentin 



Granular 
Layer 



Fig 288. — Tomes' Granular Layer. X 40. 



This, however, they do not do. They do not grow out from the cell- 
body, so to speak, but the cell recedes, leaving them behind. By this 
arrangement the terminals of the future fibers become definitely estab- 
lished, all increase in length taking place in the opposite direction, toward 
the pulp. While the active enamel-forming cells are present some little 
time prior to the odontoblasts, calcification of the enamel does not take 
place until after a definite cap of dentin has been formed, imprisoned in 
which are the terminal branches of the fibers. Therefore the fact that 
this cap of dentin is formed first, and this is not a question in dispute, 
with the fibers or cell processes securely encapsulated within it, would 



376 HISTOLOGY. 

seem to be sufficient evidence to qualify the statement that the dentinal 
fibers do not penetrate the enamel. The examination of very many 
sections of young growing teeth exhibits the fact that the early formed 
dentin and enamel will separate bodily, leaving a positive clear line of 
separation and a surface absolutely devoid of anything resembling the 
prolongation of the fibers extending from the surface of the dentin. 

Interglobular Spaces. — In that part of the dentin which immediately 
underlies the cementum numerous intercommunicating, irregularly 
branched spaces are found. These are known as the interglobular spaces 
(Fig. 288). On account of the granular appearance which this portion 
of the dentin exhibits under low magnifying power, Tomes has desig- 




Fig. 289. — So-called Interglobular Spaces in Dried Section of Dentin, x i°o. 

nated it as the "granular layer.'''' The granular layer is also found upon 
that portion of the dentin which underlies the enamel, but in this region 
it is far less marked. Many of the dentin tubes have their endings in 
these spaces. While the interglobular spaces are most numerous near 
the peripheral portion of the dentin, they are by no means confined to these 
parts. They are present in all parts of the dentin, but not so closely 
associated, and may be observed, when a dried section of dentin is exam- 
ined, as spaces with irregular outlines and sharp-pointed processes ex- 
tending in various directions (Fig. 289) . The term ' ' interglobular spaces ' ' 
becomes partly a misnomer when the so-called "spaces" are more care- 
fully examined. In normal dentin the "spaces" are filled with a soft, 
living plasma, having a structural arrangement similar to the general 



DENTIN. 



377 



matrix of the dentin, and it is only in a dried specimen that a true space 
is found by the shrinking or shriveling of the organic contents. The 
interglobular spaces forming the granular layer, which are much more 
numerous, but of smaller size, than those found in the body of the dentin, 
are also filled with a soft living plasma, and they communicate, on one 
hand, with the dentinal fibers, and, on the other, with the lacunas and 
canaliculi of the cementum. According to Sudduth, the interglobular 
spaces (so called) are occupied by masses of calcoglobulin which have 
not become fully calcified. 

Dentinification. — The dentin bulb, or papilla from which the dentin 
is forced, having already been described in Part I, the process of calcifi- 









;^* 



, 



Pulp-cells K's!.; 'SrSsJr- 58. •' c*»wr 




Odontoblasts 






<!> 



Odontoblasts 4?< n . o*~ ]3 



Calcified Dentin 



Uncalcified Dentin 



| BP> 



Fig. 290. — Pulp and Forming Dentin from an Incisor Tooth. (After Gysi.) 



cation will at once be taken up. It will be recalled that calcification of 
the dentin does not begin until the dentin papilla has developed to the 
form and size of the dentin of the future tooth-crown. When this has 
taken place, there is generated upon the surface of the papilla a modified 
form of connective-tissue cells called odontoblasts (Fig. 290). These 
cells, which are arranged in a single row upon the exterior of the papilla, 
vary in form according to their activity. When most active, they are 
broadest at the extremity directed toward the interior of the papilla. 
Proceeding from a single odontoblast there may be one or more processes, 
which are supposed to eventually occupy the tubes of the dentin, as the 
dentinal fibers. These cells each contain an oblong nucleus, which occu- 
pies the extremity of the cell most distant from the dentin, but during 



378 



HISTOLOGY. 



the period of greatest activity becomes elongated or pointed in the direc- 
tion of the process. The odontoblastic cells, while actively engaged in 
the calcifying process, are closely associated or crowded together, but 
previous to this time there is more or less space between them, which is 
filled with an indifferent tissue. The first layer of dentin being formed 
upon the surface of the papilla, it will be observed that all additions to 
its bulk take place from within (the reverse being true of enamel). 




Fig. 291. — Section through Pulp and Forming Dentin. 



As stated elsewhere, calcification of the dentin begins upon the 
coronal extremities of the crowns, the cutting-edges of the incisors and 
cuspids, and the summits of the cusps in the cuspidate teeth first receiving 
their lime-salts. While the odontoblasts undoubtedly superintend the 
calcifying process, the part taken by these cells appears to be somewhat 
indefinitely determined. It is generally supposed that the lime-salts are 
secreted under the superintendency of the odontoblasts. The secretion, 
however, does not take place around the cells, and in that way completely 



dextix. 379 

encapsule them, but around their fibrils. While this is taking place the 
odontoblasts remain free upon the surface of the pulp, and the fibrils 
assume their places as the organic dentinal fibrils. As the body of dentin 
becomes thicker, the odontoblasts are forced to recede, and in so doing 
the fibers lengthen. The dentinal tubules are, of course, formed in a 
like manner, the walls of the tube being first calcified from the secretion 
of lime-salts by the fibrils; and as the fibrils lengthen by the increasing 
thickness of dentin and the receding of the odontoblasts, the tubes also 
lengthen. 

The general character of the pulp cells at a time immediately prior 
to the appearance of the ameloblasts is vastly different from the same 
cells at maturity or after calcification of the dentin has taken place. The 
reason for this is obvious, considering that the connective-tissue mass 
does not assume its principal function until the odontoblasts are generated 
about its periphery. While at a later period the cells of the pulp are 
oblong in shape, with slender tail-like processes given off from each end, 
we find the same cells in the early embryo (sixteenth to twentieth week) 
spheroidal in outline and distributed as they continue to be, at irregular 
intervals about the semi-gelatinous matrix. (See Fig. 291.) 

When the periphery of the pulp is reached, a definite layer of cells is 
present, corresponding in every particular to those of the interior, and 
it is from these spheric bodies that the dentin- forming cells are derived. 
While the odontoblasts are usually characterized as spindle- or flask- 
shaped cells, this can only apply to the cells of later life, as those active 
at the beginning of calcification do not partake of either of these forms. 
Figure 292 (twentieth to twenty-fourth week) shows the first formed 
odontoblasts actively engaged in their function of dentinification. It will 
be observed that the cells, instead of being individualized as they appear 
at a later period, now present a racemose arrangement, such a cluster 
appearing about the entrance to each dentinal tubuli, which at this period 
are widely separated and apparently without anastomosing branches. 
The nearer the summit of the crown is approached, the less apparent is 
this grape-like association of the cells, showing conclusively that it is 
a primary condition. 

After a definite thickness of calcified dentin appears about the surface 
of the tooth-pulp, the character of the odontoblastic cells becomes mate- 
rially changed (twenty-fourth week), but even yet they do not answer 
to the description accorded them. The elongated, spindle-shaped or 
club-shaped odontoblasts are without question found in connection with 
the tissue only after calcification has progressed to a considerable extent; 



3 8o 



HISTOLOGY. 



and while it does not appear possible to detect the minute processes which 
penetrate the calcifying structure before this stage of the phenomena has 
been reached, they have nevertheless existed from the earliest inception 
of this specialized layer of cells. 

In this connection the query presents itself in regard to the manner 
in which the intercommunication between the dentinal fibers is established, 
and the probable cause for the so-called interglobular spaces about the 




■J)tv»U\». 



1MB 



Fig. 292 — Young Odontoblasts Attached to Forming Dentin. 



periphery of the dentin. The former can probably be explained by an 
examination of the peripheral cells of the pulp at a time immediately 
prior to the beginning of calcification, when it will be found that these primi- 
tive odontoblasts communicate with one another in a manner quite 
similar to the canaliculi between the lacunae of the true bone, the connect- 
ing processes being encapsuled within the substance of the calcifying 
tissue. 

Figure 293 is taken from a very thin section of a growing tooth at 



DENTIN. 



3*i 



a time in which we would most naturally look for the appearance of the 
interglobular spaces, and many such imperfections, if they be so classified, 
are observed within the substance of the newly formed tissue. 

Exceptions might be taken to the statement concerning the racemose 
appearance of the early odontoblasts previously referred to, by claiming 
the section to be one not directly through the long axis of the cells, or 




FiG. 293. — Section through Crown of Growing Tooth of Lamb. 

perhaps transversely through them. The examination of a number of 
sections, one of which is shown in figure 293, shows the cells cut trans- 
versely; the forming dentin at a appears with the tubuli squarely cut off, 
showing the dentinal fibers confined, or rather appearing as though pro- 
jecting from the lumen of the tubes. It will also be noted that the odonto- 
blasts are irregular in outline, some of them being almost hexagonal, and 



3 82 



HISTOLOGY. 



as the calcified tissue is approached, they gradually become reduced in 
size and much modified in contour. 

After a dentin cap or matrix of considerable thickness has made its 
appearance and the enamel cells are about to assume their functional 
activity, the odontoblasts for the first time begin to resolve themselves 
into the elongated flask-shaped cells, thus answering the description 
usually accorded them, as illustrated in figure 290. In fact it would 




Fig. 294. — Odontoblasts in Transverse Section. 



appear that they assume this shape only when the actual lime deposit 
begins. If we examine the line of union between the dentin and enamel 
during the early growth of these tissues, it will be ascertained that, not- 
withstanding the dissimilarity of the two structures at maturity, there 
appears at this line of junction a matrix which may be differentiated only 
by the free extremities of the ameloblasts, as shown in figure 295. When 
the enamel matrix begins to form, a faint line of demarcation between 
the two may be observed, the difference in the appearance of the ground- 



DENTIN. 



3*3 



work of the two structures being one brought out by differential staining, 
that of the enamel taking the darkest stain. 

It will be observed, therefore, that after the dentin germ has assumed 
the exact size of the dentin of the future tooth, certain cells appear upon 
its periphery, and under their superintendence a definite layer of dentin 




Fig. 295. — Section through Pulp, Dentin, and Forming Enamel. 
A, Calcified dentin; B, Pulp; C, Ameloblasts. 

soon results. This first formed layer of dentin is definite and unchange- 
able in location, and it has within its substance the minute processes from 
the dentin-forming cells which are destined to become and really are the 
terminals of the dentinal tubules. All who have given the subject of 
dentin calcification careful consideration are practically agreed as to the 
part which the peripheral pulp-cells play in the process. This is to the 



384 HISTOLOGY. 

effect that not about the body of the cells themselves, but around their 
processes the lime salts are deposited. After a distinct layer of specialized 
cells has become fully established upon the very periphery of the papilla, 
the first change which takes place is a slight withdrawal of these cells 
from this point, leaving behind slender hair-like processes which occupy a 
portion of the space previously taken up by them, and about the ex- 
tremities of the cells and their processes which are directed toward the 
enamel organ calcified material is generated. Zone upon zone of calcified 
dentin appears in this way, the body of the cell receding, leaving in its 
wake its processes encapsuled within the calcified structure as the dentinal 
fibers. 

In connection with the primitive layer of dentin-forming cells, there 
are usually described lateral processes passing from cell to cell, apparently 
serving the purpose of communication between the cells. But these 
have recently been shown to be simply a network of connective-tissue 
fibers supporting the body of the cells. The theory of Andrews, brought 
out some years ago in regard to the specialized layer of pear-shaped 
cells — dentin corpuscles, as he termed them — may be accepted, and 
these should be considered as having something to do with the process of 
dentinification. The presence of these pear-shaped cells at the beginning 
of calcification and during the continuance of this process can be easily 
demonstrated, and if we accept them as being concerned in the process 
of dentin formation, they might in a measure modify the function now 
accorded the elongated club-shaped cells, the odontoblasts. It is ques- 
tionable whether the odontoblasts alone are reponsible for the growth 
of dentin; they undoubtedly control the actual process of lime deposit, 
but the additional cells, no doubt, contribute to the structural make-up 
of the tissue. It may be that by. modifying certain parts of the matrix, 
the result in the general structure is the dentinal sheaths; this part of the 
tissue being so markedly different from the bulk of the intercellular sub- 
stance would lead us to believe that it was developed from specialized, 
cells. Further, it is said that while the dentinal tubules are filled with 
a living substance, this substance is not solely the product of the processes 
of the odontoblasts. That there is a special distribution of non-medul- 
lated nerve terminals as well as a rich plexus of blood-vessels about the 
periphery of the pulp is unquestionable, and this supply is just as plentiful, 
or perhaps more so, at maturity as it is at the beginning of calcification 
when the dentin cells are most active. From this we might be led to 
believe that this special blood and nerve supply to the periphery of the 
pulp is not solely for the upbuilding of the dentin and therefore distributed 



CEMENTUM. 



385 



to the peripheral cells, but also for the permanent welfare of the resultant 
tissue, this being brought about by some circulatory system throughout 
the tubules of the dentin. 

Cementum (Fig. 296). — Investing the roots of the teeth is a sub- 
stance which, both chemically and physically, is closely allied to bone. 
This external covering is known as the cementum, and while generally 
regarded as being confined to the roots of the teeth, by some it is considered 



Lucunae 




Canal- 
iculi • 



Fig. 296. — Cementum from Root of Molar. X 200. 



to extend to and completely invest the crowns during the early part of 
their existence, in this latter location being known as the enamel cuticle, 
or membrane of Nasmyth. 

Generally speaking, the cementum begins by a thin margin at the 
neck of the tooth or cervical line. It may commence at the free enamel 
margin of the crown, or it may slightly overlap this structure. It is 
thinnest at the neck of the tooth, and gradually increases in thickness as 
the apex of the root is approached. In teeth with closely associated 
25 



3 86 



HISTOLOGY. 



roots the cementum frequently extends from one root to the other, result- 
ing in a firm, osseous union. Histologically considered, the structure of 
cementum, like ordinary bone, consists of a gelatinous, basal substance, 
combined with the salts of lime, and of numerous little hollow spaces — 
lacunce. Branching in every direction from the lacunae are many minute 
processes — canaliculi. 

The Matrix. — The matrix is so nearly identical with that of bone 
that it is with difficulty that they can be distinguished. By decalcification 
it retains its form and structure, and by the intimate blending of organic 



ISMHMti 




Fig. 297. — Longitudinal Section through Root of Human Molar. 
Incremental Lines of Cementum. X 30. 



and inorganic substances it is provided with hardness, solidity, and 
elasticity. Calcium salts and collagenous fibrils, united by a small 
amount of cement-substance, in finer or coarser bundles, compose the 
ground-substance, or matrix, of cementum. 

Let us first take up the study of this tissue at different periods of its 
existence, and in this manner learn of its character, its mode of develop- 
ment, and the changes which take place as its growth proceeds. The 
striated markings of the tissue have led to the belief that there are, during 
the process of cementification, periods of activity and periods of rest or 
little activity. An examination of the structure under low power (Fig. 
297) shows the incremental lines placed, with more or less regularity, one 



CEMENTUM. 



387 



beyond the other, and when thus studied adds much to the strength of the 
theory of interrupted development. 

Figure 298 is prepared from a developing deciduous incisor three 
months after birth. At this period the developing organ is made up of 
enamel and dentin alone, the process of cementification not yet being 
under way. The establishment of the dentinal periphery, which surface 
is unchangeable, provides a basis for the first layer of cementum generated 
by the cementoblasts, which at this period are forming about the inner 
wall of the tooth-follicle. In close proximity to the surface the inter - 



Enamel 



Surface of 
Dentin 




^ .J- 







*<.-.•: 



>v 



Fig. 298. — Section through Developing Incisor, Three Months after Birth. X 30. 



globular spaces are observed somewhat widely distributed, and propor- 
tionately large in size, resulting in a surface poorly calcified and forming a 
ready attachment for the cemental tissue. Figure 299 .shows the process 
of cementification under way, the section being prepared from a six- 
month-old tooth. In an examination of the ground-substance of this 
developing tissue there is an unbroken granular appearance, possessing 
neither striations, fibers, nor cement-corpuscles. This appearance is 
one which persists in the oldest or first-formed stratum, and is again 
noticeable in the outermost or youngest stratum. While the oldest 
stratum or strata retain this primary character, this cannot be said of 



3 88 



HISTOLOGY. 



those subsequently laid upon it, for they successively develop in their 
matrix the partially calcified cells and fibers from the formative tissue. 

Figure 300, taken from a one-year-old tooth, shows a further advance 
in the process of cementification. Many of the transverse fibers of the 
alveolodental membrane are observed penetrating the developing tissue, 
and will, at a later period, by their partial calcification, become a part of 
its substance. Already there has been established an intimate blending 
of the cemental tissues with the dentinal tissues through the medium of the 
granular layer, and by the further calcification of the latter this union 






I 



/v*.t< 




Fig. 299. — Developing Cementum, from Six-month-old Tooth. X 200. 
a, Developing Cementum; b, Granular Layer; c, Terminals of the Dentinal Tubuli. 



gradually becomes more thorough. Figure 301 illustrates three distinct 
zones of developing cementum; the older unbroken granular zone at A, 
now beautifully cemented to the granular layer; a second or intermediate 
zone, B, having encapsuled within its ground-substance many of its 
formative cells; and an outer zone, C, but recently laid down, showing 
numerous, longitudinal, wave-like striations, emblematic of the cemen- 
toblastic activity. In this outer zone the minute laminations disappear 
as the tissue becomes more thoroughly calcified and the matrix gradually 
partakes of the nature of the older tissue. 



CEMENTUM. 



389 



The position occupied by the cementum on the root has much to do 
with its character. In the region of the cervix the cement-corpuscles 



Fibers of 
Cementum 



Older Strat 




ranular 
Layer 



-.v 

Fig. 300. — Section through One-year-old Tooth. X 60. 





Fig. 301. — Developing Cementum, from Transverse Section of Bicuspid. X 100. 

are few in number, and when present possess extremely short and irregular 
processes. In the region of the apex the structure is much more complex 



39° 



HISTOLOGY. 



in character, longitudinal striae, transverse fibers, cement-corpuscles, 
and zones of apparently unbroken granular matrix all serving to this end. 

To continue the study of this tissue let us examine in detail the 
lamellae, the cement-corpuscles, and the cement-fibers. 

The Lamella. — We are told that the lamellae are about the same in 
number over all parts of the tooth-root, but that they are much thinner 
at the neck than at the apex. In addition to this they are usually con- 
sidered as running parallel, or nearly parallel, to the surface of the dentin. 
While these statements might, and probaby do, describe the disposition 




i ; 11 ^ • i in i— i * T 

\fafc - - 71 ~- c -» j . ■■+ ■<* L.T 








Fig. 302. — Transverse Section from Root of Bicuspid, showing Variation in the 
Disposition of the Lamellae. X 40. 

of the lamellae in young cementum, they do not apply with so much 
certainty to the conditions after the adult period. The lamellae in the 
region of the apex are not only of greater width, but are usually greater 
in number than those occupying the cervix of the same root. 

Figure 302 is prepared from a transverse section of an adult bicuspid 
in the region of the apex, and shows how the disposition of the lamellae 
may vary in thin, normal cementum. At A, which represents the granular 
union of the cementum with the dentin, the incremental lines are ob- 
served to follow the surface of the dentin. As the center of the area is 
approached this regularity is much interfered with, some of the lamellae 
being discontinued, others greatly thickened, while the field, taken in its 



CEMENTUM. 



39 1 



entirety, exhibits anything but regularity in the laying down of the 
different strata. This same condition may be observed in longitudinal 
section. While the lamellae are usually characteristic of the cemental 
tissue in general, they are seldom found in interdentinal cementum, or 
that growth which takes place between roots, resulting in their fusion 
(Fig. 303). This, of course, refers to the tissue as formed between closely 
associated roots of an individual tooth, and not to that union which 
sometimes takes place between the roots of different teeth. The inter- 
dentinal tissue previously referred to appears to have many characteristics 



Interdentinal 
Cementum 




Fig. 303. — Transverse Section through Fused Roots of Molar Tocth, showing 
Interdentinal Cementum. X 30. 



common^to itself; thus, the cement-corpuscles are peculiar in form, fibers 
are few in number, and, as before stated, the lamellae are not decided. 
Cement-corpuscles. — Many of the cementoblasts of the peridental 
membrane, like the osteoblasts of the periosteum, become encapsuled 
within the developing tissue, and persist as irregularly shaped spaces, 
filled with a protoplasmic mass, and are known as cement-corpuscles. 
These correspond to the lacuna: oj bene, but, unlike these, are very 
variable in size, in form, and in the number and direction of their proc- 
esses. Figure 296 shows a number of cemental lacunae and canaliculi. 
In the majority of instances the body of the corpuscle will be found to be 
oval or slightly oblong, with its long axis parallel to the surface; but it is 
by' no means uncommon to find them very irregular in outline, with the 



392 



HISTOLOGY. 



greatest diameter in the opposite direction. The processes are quite 
variable in length and irregular in their course, and, while there is a 
general disposition for them to extend toward the surface, they in many 
instances radiate in various directions. All of these features are in con- 
tradistinction to the lacunas and canaliculi of bone, which are placed 
with much more regularity in the osseous matrix, the corpuscles being 
oblong or cylindric in outline, with their processes about equally distrib- 
uted in every direction, and uniting directly and positively with the 
canaliculi of neighboring lacunae. As previously stated, the cement- 



• /C^vv , 










Fig. 304. — Cement-corpuscles of Outer or Younger Strata. X 40. 



corpuscles are very variable in outline, this difference in form appearing 
to be much influenced by the part of the tooth examined. The younger 
corpuscles (Fig. 304), or those associated with the outer strata, are usually 
distinctly outlined and provided with delicate processes, the majority of 
which are directed toward the surface. In the older strata the outlines 
of the corpuscles are much more irregular, the processes short and 
extremely clumsy. 

The proportionate distribution of the corpuscles to the various parts 
of the tooth-root is as follows: The innermost or oldest zone and the 
outermost or youngest zones contain but few; in the intervening strata 
they are most abundant, especially in the region of the apex, becoming 
less numerous in passing crownward. In interdentinal cementum the 



CEMENTUM. 



393 



corpuscles are somewhat regularly distributed throughout the ground- 
substance adjacent to the granular layer, but near the center of this 
confused mass of imperfectly calcified tissue they are seldom present. 
When the interdentinal space is slight, peculiarly formed corpuscles are 
often observed (Fig. 305), provided with a long, rod-like, central portion 
or trunk, from which are given off numerous tree-like branches, the 
terminals of which are frequently lost in the granular layer upon either 
side. 

Cement-fibers. — In a manner similar to that in which the cemento- 
blasts become encapsuled within the developing cemental tissue forming 







■'-- Y •»'f #-'4 V^i 




Fig. 305. — Cement-corpuscles Common to Interdentinal Cementum. X ioo. 



the cement-corpuscles, many of the fibers of the peridental membrane 
undergo a like transformation, and are found in the tissue as more or less 
imperfectly calcified fibers transversely disposed. By many writers these 
filamentary, thread-like structures have been compared to the delicate, 
net-like processes which pass through the concentric lamellae of bone, 
serving to hold them together and designated as Sharpens fibers; but, 
according to Black, these are the principal fibers of the alveolodental 
periosteum, and, as already stated, become a part of the cemental tissue 
during its evolution. In figure 306 the fibers are shown under high 
power; A represents the primary or older stratum of the tissue, and it is 
from the outer margin of this zone that the fibers first make their appear- 
ance, passing more or less directly in the direction of the surface until the 



394 HISTOLOGY. 

next incremental line is reached, at which point they gradually disappear, 
but recur in the succeeding lamellae. There is a marked disposition for 
the fibers of each concentric lamella to keep within its borders, or, in 
other words, to become individualized; but in many instances they pass 
through from one lamella to another, and occasionally extend unbroken 
through the entire thickness of the tissue. It occasionally happens that 
the fibers are plentifully distributed to a region comprising three or four 
lamellae, followed by a zone of similar proportions in which they are 
entirely absent. The cement-fibers, considered as the partially calcified 
residue of the principal fibers of the peridental membrane, would naturally 




Fig. 306. — Transverse Section through Root of Molar, showing Cemental Fibers. X 300. 

assume a general direction relative to their manner of distribution before 
this change had taken place, and in most instances they are thus disposed. 
In figure 307, taken from the center of a long axis of a growing bicuspid, 
the disposition of the fibers, which are alone observed in the second 
lamella, is slightly crownward. The inclination for the fibers to be thus 
disposed is most pronounced in young cementum, but after middle life, 
or at a period when the tissue has greatly increased in thickness, the 
course of the fibers, even in the same locality, is greatly at variance. 

In figure 300, also from a young tooth, the fibers are shown springing 
directly from the peridental membrane, with their free extremities 
penetrating this tissue. This illustration is prepared from a transverse 
section in the cervical region, and the inclination of the fibers is such as 



CEMENTUM. 



395 



to warrant the belief that they were some of those whose function it has 
been to return the tooth to its normal position when slightly rotated upon 
itself. Another class of fibers common to the cement-tissue are those 
which appear to be grouped in bundles, springing more or less regularly, 
at intervals, from the granular layer and penetrating the basement layer 
of the cementum as though serving to tie this tissue to the periphery of 
the dentin. In figure 308 a number of these bundles are shown at A, B, 
and C. While the field is but a small proportion of the circumference 
of the root, they are observed, under low power, to be distributed in a 




like manner to all parts. These circumferential fibers, as they may be 
called, are also observed in longitudinal section, being distributed with 
considerable regularity throughout the whole extent of the root. They 
are also present in the tissue at the earliest period at which its character 
may be studied, the individual bundles at this time being proportionately 
larger. These might be, and probably are, considered as prolongations 
from the dentinal fibers, but it is doubtful if the true fibers of the dentin 
are ever found penetrating the cementum. 

Cementification. — We have seen in the study of the development of 
the teeth, that the tooth-generating organs were confined in a closed sac 
or follicle, and while the walls of this sac were not directly interested in 
the calcification of the dentin or enamel, this cannot be said of the 
cementum. Attention has also been directed to the fact that at the time 



396 



HISTOLOGY. 



of the eruption of the crown of the tooth a portion of the root only is 
calcified. As the growth of the root continues, the follicular wall becomes 
closely adherent to it. Upon the inner face of this vascular membrane 
a layer of osteoblastic cells (cementoblasts) is generated, and as a result 
of the calcification of these cells the cementum is formed. It will thus be 
seen that the process of cementification is but a slightly modified form of 
subperiosteal bone development. At the beginning of cementum cal- 
cification the diameter of the dentin of the root is as great as it will ever 
be, all additions to its bulk taking place from within. But while the 




Fig. 30S. 



diameter of the dentin is thus fixed, the diameter of the root is increased 
by the additional layers of cementum as they are deposited upon its 
surface. As previously stated, a single layer of cementoblasts is first 
formed in the membrane surrounding the root, these soon becoming 
inclosed in a spherule of lime. By the time this has taken place another 
layer makes its appearance, assuming all the characteristics of the first 
formed layer. Other layers are formed in turn until the cementum 
assumes its mature thickness. 

The Dental Pulp (Fig. 309). — The tooth-pulp, or formative organ 
of the dentin, occupies the central or pulp-cavity, and in the fully developed 
tooth assumes a general outline closely corresponding to the exterior 



THE DENTAL PULP. 



597 



of the organ.* Along with its primary function of generating the dentin, 
it becomes the medium through which this structure receives its vascular 
and nervous supply. 

Histologically considered, the pulp may be described as a mucous- 
like, protoplasmic matrix, containing delicate connective-tissue fibers 
not formed into bundles and numerous nucleated cells, the latter being 
especially numerous on the periphery of the pulp, or that portion which 
comes in contact with the dentin. The cells are not closely enough 



Enamel 



Cementum 




Dentin 



Pulp 



Blood-vessels 



Fig. 309. — Longitudinal Section through Human Cuspid, showing Tooth-pulp. 

{After Gy si.) X 10. 



associated to form a complete tissue in themselves, but are found em- 
bedded in a mucoid matrix, with always a definite space between them. 
In general the cells are elongated or spindle-shaped, with a delicate, 
hair-like process attached to either extremity. In the pulp-chamber the 
cells vary somewhat in outline, in some instances being spheroid, in 
others appearing as slender filaments, so that the cell proper can 
scarcely be distinguished from its processes. A third class of cells 

*The pulp not only occupies the central cavity in the tooth-crown, but the canals of the 
roots as well; therefore the form of the pulp corresponds to the outline of the pulp-cavity, 
already described. 



39« 



HISTOLOGY. 



may be met with, from which three or more filaments are given off. As 
stated, the distribution of cells varies considerably in different parts of 
the pulp, this being true not only as regards numbers, but also as to the 
relations existing between the cells. In the coronal portion of the pulp 
the position assumed by each individual cell appears to be without regard 
to the position of neighboring cells, while in that portion of the pulp 
occupying the root-canals the cells are arranged parallel with the length 




Fig. 310. — Transverse Section through Pulp. Blood-vessels and Nerves in Cross Section. 



of the root. The cells are least in number in the interior of the pulp, 
but gradually become more plentiful as the periphery is approached. (See 
Cells of Dentin Papilla, page 257). 

The Odontoblasts (Fig. 311). — The most active cells of the pulp are 
those directly on its periphery, in contact with the dentin, and known 
as the odontoblasts. The odontoblastic layer, otherwise known as the 
membrana eboris, is composed of a single row of cells, each of which 
contains, near the extremity most distant from the dentin, a well-defined 
nucleus. They are large, elongated cells, each furnished with three sets 



THE DENTAL PULP. 



399 



of fibers or processes — the dentinal process, the pulpal process, and the 
lateral process. The dentinal process or processes — there may be more 
than one present — communicate with the deeper-lying cells of the pulp, 
while by means of the lateral processes the cells are brought into com- 
munication with neighboring cells. The processes given off in the 
direction of the dentin, or the dentinal processes, may be one for each cell, 
in which case they are of considerable size, and are inclined to taper as 




Dentin 



Odonto. 
blasts 



Blood- 
vessels 



Fig. 311. — Pulp and Dentin in Longitudinal Section. 

they enter the substance of the dentin. Again, a single cell may give off 
a number of smaller processes in this direction. The odontoblasts vary 
much in form according to their functional activity. Before the period 
of dentinification they are spheroid or pyriform, during the period of 
calcification the dentin extremity becomes somewhat flattened and 
square, while in advanced years they again return to their primitive, 
rounded form. Covering the entire surface of the pulp like an epithelium, 
the odontoblasts are especially closely associated at the end nearest the 
dentin, forming an unbroken layer, while the pulpal extremities are 
inclined to assert their individuality by disassociation. 



400 



HISTOLOGY. 



Blood-vessels of the Pulp. — The pulp is richly supplied with blood- 
vessels, forming networks extending principally in a direction parallel to 
the long axis of the tooth, and finally terminate in a capillary plexus 
closely associated with the odontoblastic layer. The veins of the pulp 
are ordinarily somewhat larger than the arteries, and form numerous 
anastomoses. This organ appears to be destitute of lymphatics — at 




Fig. 312. — Section through Pulp. Fig 311 in Transverse Section. 

least, none are known to occur in its substance. The blood-vessels of the 
pulp are provided with a longitudinal layer of thinly distributed muscular 
fiber, but otherwise the walls of the vessels are noted for their delicacy. 

Nerves of the Pulp. — After entering the apical foramen either by one 
large trunk or by two or more minute ones, the fibers pursue a parallel 
course, breaking up but little or giving off but few fibers in that portion 
of the pulp confined to the canal. When the expanded or coronal portion 
of the pulp is reached, numerous subdivisions occur which are distributed 
in every direction, and ending in a rich plexus beneath the odontoblastic 
layer, or membrana eboris. In the body of the pulp the fibers are 



THE DENTAL PULP. 



40I 



medullated, but those occupying the periphery are non-medullated and 
supposed to pass into the dentinal tubes. While this latter hypothesis 
is in all probability correct, such a distribution of the germinal fibers has 
never been definitely demonstrated. Two investigators (Ball and 
Magitot) claim to have partially satisfied themselves in regard to the final 
distribution of the non-medullated fibers. The former states that he has 
traced these fibers into continuity with the larger medullated fibers in the 
deeper pulp-tissue, and claims to have found them passing through the 



Main Nerve- 
trunk 



Main Blood- 
vessels 




Branching of Main 
Nerve-trunk into 
Single Fibers 



Branching of Main 
Blood-vessels into 
Capillaries 



Fig. 313. — Distribution of Blood-vessels and Nerves to the Pulp of Human Molar. 

{After Gysi.) X 20. 



membrana eboris, beyond which point they assumed a direction parallel 
to the dentinal tubules. This theory is controverted by Magitot, who 
claims that the dentinal fibers are, in a measure, themselves prolongations 
of the nerves, being so constituted through the medium of the branched 
stellate cells which lie immediately beneath the membrana eboris, and 
by which the nerves are made continuous. 

Nasmyth's Membrane. — Nasmyth's membrane, otherwise known 
as the enamel cuticle or persistent dentinal capsule, is an exceedingly thin 
and peculiarly indestructible structure, entirely covering the enamel. 
As to the presence of this membrane, which can be demonstrated only by 
chemic detachment, there appears no doubt, but in regard to its origin 
and definite structure much difference of opinion has been expressed. 
26 



402 



HISTOLOGY. 



By some writers (Tomes and Magitot) it is maintained that it is continuous 
with, and similar in structure to, the cementum covering the root, being 
an extension of the outermost layer in the region of the neck of the tooth; 
and, in view of the fact that lacunas are found in its substance, this theory 
would appear to be correct. On the other hand, it is considered to be a 
product of the epithelium (Huxley and Kolliker) and in no manner con- 
nected with the cementum. In the opinion of the author, it would be 
difficult to understand how the theory advanced by Tomes could be 
accepted. During the entire period of saccular development the crown 



Alveolar Wal 




Cementum 




Alveolodental 3BH 

Membrane BB 



Blood-vessels of 
Alveolodental Mem- 
brane in Trans- 
verse Section 




rS*^ 



Dentin 



Fig. 314. — Transverse Section through Root of Human Incisor and Surrounding 
Alveolar Wall, with Alveolodental Membrane Intervening. X 40. 



of the tooth is in close relationship to the enamel organ, this structure 
intervening between the forming enamel and the wall of the tooth-sac, 
from which the cementum is developed. It would, therefore, appear 
that this membrane is generated from the external epithelial layer of the 
enamel organ by a change in the character and form of these cells. Sud- 
duth attributes its formation to a metamorphosis of the ameloblastic 
layer, the prismatic cells assuming a horizontal direction. The amelo- 
blasts are observed to be prismatic in form up to the point at which the 
enamel prisms are yet unfinished, but as the surface is approached they 
are observed to shorten and widen, and near the gum-margin they 
assume a longitudinal direction instead of being at right angles to the 



ALVEOLODENTAL MEMBRANE. 



403 



body of the crown. Mrs. Emily Whitman has devoted much time to the 
study of the development of mammalian teeth, and appears to be of the 
opinion that the cuticula dentis is the result of a change in the form and 
character of the enamel cells, this metamorphosis taking place either 
before or after calcification of the underlying tooth-tissues. Nasmyth's 
membrane shows many characteristics which differ from those of the 
body of enamel subjacent to it, serving as an indestructible, highly 
polished surface-capping to the enamel prisms. The indestructible 
nature of this membrane by reagents would appear to indicate that in 




Fig. 315. — Section through Root of Tooth, Alveolodental Membrane, and Alveolus 
a, Alveolus; b, Blood-vessel; c, Alveolar Portion; d, Dental Portion; e, Cementun . 



structure it is closely akin to the structure lining the dentinal tubules, 
the lacunae, etc. 

Alveolodental Membrane (Figs. 314 and 315) . — As a general descrip- 
tion of this membrane has already been given in Part I, it alone remains 
to treat of its histologic character, which may best be accomplished by first 
referring to the duties which it has to perform. These may be divided 
into three classes — functional, physical, and sensory. The functional 
office is accomplished through its cellular elements — the osteoblasts and 
cementoblasts; the physical office is performed by the fibrous elements, 
through which the tooth is fixed in its position; and the sensory office 



404 



HISTOLOGY. 



through the abundance of nerves, which are richly distributed to all parts 
of the membrane. We, therefore, find in this structure, besides con- 
nective tissue, cells, fibers, nerves, and blood-vessels. The principal 
cells, as already stated, are the osteoblasts and cementoblasts, but there 
are also present fibroblasts and osteoclasts. 

The osteoblasts, which are instrumental in the upbuilding of a portion 
of the alveolar walls, are found lying against the bone, between the 
principal fibers. These cells do not appear to be evenly distributed, 
being numerous and crowded together in some parts, while others will 



Epithelium 



Connective 
Tissue of 
Gum 



Alveolar 

Wall 




Alveolo- 
— dental 
Membrane 



Cementum 
of Root 



Fig. 316.— Transverse Section through Root of Tooth, Alveolodental Membrane, Thin 
Wall of Alveolus, and Gingival Tissue. 



appear to be almost destitute of them. They are most plentiful in the 
young subject, and seldom present at all in old age. In youth the 
alveolodental membrane is thickest, and, as the building of bone occurs 
on the inner wall of the alveolus, it can only progress as the membrane 
becomes reduced in thickness. The osteoblasts are polygonal cells, in- 
clining to the oval form, and vary greatly in size, with their longest 
diameter at right angles with the surface of the forming bone. During 
the period of the development of the young alveolar wall they are inclined 
to be crowded together, and are frequently much distorted from pressure 



ALVEOLODENTAL MEMBRANE. 405 

upon one another. As age advances this condition becomes less pro- 
nounced, and the cells separate into groups. 

The Cementoblasts. — Stationed upon the opposite side of the mem- 
brane, or that in contact with the root of the tooth, are another class of 
cells — the cementoblasts — or those cells which are concerned in the 
formation of the cementum. Like the osteoblasts, these cells are found 
lying between the principal fibers of the root-membrane. They differ 
in form from the osteoblasts, notwithstanding that they have a similar 
function. Instead of the polygonal form common to the osteoblasts, we 



Pulp 



Dentin 



Gingival 
Tissue 




Fig. 317. — Longitudinal Section through Root of Growing Tooth near the Cervix. 

find these cells to be more or less flattened, with outlines somewhat 
irregular. Extending from the body of the cell, which contains a well- 
defined nucleus, are a number of irregular processes, which penetrate 
the neighboring fibers or the interfibrous substance. Unlike the osteo- 
blasts, the cementoblasts appear at all times to be evenly distributed 
over the surface of the cementum, occupying all the space except that 
taken up by the fibers as they leave the cementum. As to the develop- 
ment of the osteoblasts and cementoblasts, they appear to be carried to 
the fibrous meshes of the membrane by the blood as leukocytes or ameboid 
cells, after which, by differentiation, they become fitted for the develop- 
ment of bone or cementum, assuming their respective places against 
the surface of one or the other of these structures. 



406 HISTOLOGY. 

Fibroblasts and Osteoclasts. — Fibroblasts and osteoclasts are also 
present in the alveolodental membrane, the former for the purpose of the 
increase or renewal of the fibrous tissue, the latter being functionally 
concerned in the removal of a part of the alveolar walls to accommodate 
the ever-varying position of the teeth, or acting in a similar manner upon 
the cementum of the root. The osteoclasts, or giant-cells, are generally 
inclined to the round or oblong form, and usually contain a number of 
nucleoli. They vary much in size, and are seldom branched or provided 
with processes. In addition to the four classes of cells already mentioned 



Cementum 



Blood-vessel 



Alveolodental 
Membrane 



Gingivae 




Fig. 318. — Longitudinal Section through Alveolodental Membrane, Gingival Tissue, 
and Root of Tooth. Cervical District. 

as being present within the meshes of the fibrous tissue of the root-mem- 
brane, there is another class, present, however, during youth only, which 
appears to be in course of development, and, therefore, without apparent 
function. 

The Fibers of the Alveolodental Membrane. — The principal fibers of 
the alveolodental membrane are those which extend from the cementum 
on one side to the alveolar wall on the other, and become firmly fixed 
at either extremity by penetrating the calcified structures. The fibers 
are all of the white, or inelastic, connective-tissue variety. It is by means 
of the connective-tissue fibers that the actual attachment of the membrane 



ALVEOLODENTAL MEMBRANE. 



407 



both to the bone and to the cementum takes place, the fibers passing 
directly into the hard tissues, which they traverse for some distance, 
being here known as Sharpens fibers. 

The arrangement of the fibers is somewhat different over the various 
parts of the root. In the region of the gingival margin they pass out 
from the substance of the cementum, retaining their solid form or dividing 
into fasciculi of finer fibers. In general the fibers lie parallel with one 




Fibers 



Fig. 319. — Section showing Fibers of Alveolodental Membrane, Attached to and Passing 

Out from the Cementum. 



another, deviating only to give place to blood-vessels and nerves. There 
is some variation in the distribution of the fibers about the different 
gingival surfaces. Upon the labial and lingual surfaces they pass out 
directly into the fibrous tissue of the gum, and soon become lost in this 
tissue. On the mesial and distal surfaces the fibers passing the lower 
margin of the alveolar wall join the fibers of the neighboring tooth. 
This disposition for the fibers to bend toward the adjacent tooth is first 
observed at the various angles of the gingival margin. All about the 



408 



HISTOLOGY. 



free border of the gum the fibers from the alveolodental membrane 
assist in forming this tissue, which is covered by a dense epithelial coating 
of moderate thickness, surrounded or surmounted by the peridental 
fibers. As the border of the alveolar wall is approached, the fibers are 
observed to pass under the proper tissues of the gum, and unite with the 
outer periosteal layer overlying the outer alveolar wall. The fibers 
immediately within the alveolus are slightly inclined in an apical direction, 
while those occupying the central portion of the membrane, or that 
midway between the apex and the gingiva, pass nearly straight across 



Alveolus 



Cementum 



Pulp 



Dentin 




Fig. 320. — -Transverse Section through Growing Roots and Alveolus. 

from the cementum to the bone. It is in this locality that the largest 
and strongest fibers are found. As the apex of the root is approached 
the inclination of the fibers is crownward from the cementum to the 
alveolar wall. In this situation the single fibers are inclined to break up 
the fasciculi. Immediately surrounding the apex of the root the fibers 
are irregular during youth, but are disposed more regularly or fan-like 
in older subjects. 

While this account briefly furnishes a description of the distribution 
of the fibers in various locations, and is in most instances correct, they 
occasionally vary from this arrangement. While in most respects the 
fibers of this membrane closely resemble the corresponding fibers of 



ALVEOLODENTAL MEM3RANE. 40Q 

attached periosteum, they possess some peculiarities. It might be 
supposed that the fibers passing out from the cementum would in some 
way differ from those springing from the alveolar wall, but, with the 
exception of being somewhat less in size, they are otherwise of the same 
character. 

Interfibrous Elements. — Besides the various forms of cells, blood- 
vessels, and nerves, there is present in the alveolodental membrane an 
interfibrous tissue. This tissue is principally composed of the fibroblasts 
belonging to the principal fibers, and other fibroblasts accompanied by 
delicate fibers which appear to be independently distributed. This 
interfibrous tissue, which is thus seen to be ordinary fibrous connective 
tissue, appears to pervade the entire membrane wherever sufficient space 
is found to permit of its presence. In some parts of the membrane this 
tissue appears to be more plentiful than the principal fibers themselves. 
The interfibrous tissue also forms an investment for the blood-vessels 
and nerves in addition to the tissues properly belonging to their walls. 



4io 



HISTOLOGY. 




Fig. 321. — Evolution of the Face. {After Haeckel.) 



CHAPTER VII. 




Embryology of the Mouth and Teeth. 

The nourishment required for the growth of the embryo and its 
maintainance during the earliest part of its development, and in higher 
animals during the whole of this period, is supplied either from the 
mother by means of a placenta, or it is drawn from the supply of concen- 
trated food-material, stored up for that purpose in the form of yolk in 
the egg. The formation of the future digestive organs of the mature 
organism begins, however, already at a very early stage of the development 
of the embryo. The essential epithelial parts of the digestive canal 
are derived from the entoderm, while the more auxiliary parts, such as 

muscles, connective tissue, blood-vessels, etc., 
which are not present in the lower animals, 
but become more and more conspicuous as we 
advance in the scale of animal organization, 
are derived from the visceral layer of the 
mesoderm. At first, the primitive gut presents 
a shallow, wide groove, but it becomes grad- 
ually deeper by a folding up of the sides; the 
edges approximate more and more, and when 
finally a union has taken place, there is a tube 
formed — the so-called primitive digestive tract. 
While the latter still has for a time an outlet 
in the form of the vitelline or umbilical duct, 
the tube is closed anteriorly, as well as pos- 
teriorly. As development proceeds, there becomes noticeable a depression 
of the ectoderm at the anterior end of the tube, opposite the ventral side of 
the latter, and here ectoderm and entoderm lay closely attached to each 
other, forming a thin membrane. This point of contact of the two layers 
is known as the oral plate. Soon the plate ruptures, and thus an anterior 
aperture of the gut, the oral sinus or primitive mouth, is formed. 

The gradual transformation of the primitive aperture into the 
permanent mouth and face is brought about mainly by the development 
and transformations of its boundaries, and are, in brief, the following: 
About the end of the third week, a series of conspicuous elevations or 

411 



Fig. 322. — Diagram showing 
the relation between Ectoderm 
and Entoderm in the Mouth of 
a Mammalian Embryo. 

a. I. and p. I., Anterior and 
posterior lobes of the hyphysis; 
m. t., medullary tube; ph., 
pharynx; o.p., oral plate; x 
and y, ectoderm which pro- 
duces the lip and teeth of the 
lower and the upper jaw, re- 
spectively. (Stohr.) 



412 



HISTOLOGY. 



processes are developed around the opening, from the front and the sides. 
The one coming down from the head is called jronto-nasal process; those 
coming from the sides are arranged in form of parallel bars and are 
known as arches. There are five pairs of arches; in fishes they give rise 
to the formation of the gills, and the individual pairs are separated from 




Fig. 323. Fig. 324. 

Fig. 323. — Head of a Young Dogfish. (Slohr.) 

Fig. 324. — Head of a Human Embryo of 10 mm. It Illustrates the Phylogenetic Rela- 
tion of the Visceral Arches. (Slohr.) 
g.c, Gill cleft; m, mouth; n, nasal pit; c.s., cervical sinus; g. c 2 , second gill cleft; h, hyoid 
arch; md., mandibular arch; sp., (spiracle) auditory groove. 

one another by slits. In human embryos there is no communication 
between the inside and outside of the arches, but there are distinct 
furrows which serve as lines of demarcation between the individual 
bars. On account of these formations having connection genetically 
with air-absorbing viscera, the following names are given: Visceral or 
branchial arches, inner visceral furrows or pouches, external visceral 
furrows or clefts. From the accompanying illustrations it can be clearly 




Median Portion 
of the Palate 




Dental Ridge 

Lateral Portion 
of the Palate 



Fig. 325. Fig. 326. 

Fig. 325 and 326 Present the Development of Various Parts of the Palate. (After His.) 



seen that only the frontal process and the first visceral arch participate 
in the formation of the permanent mouth and face; the other four arches 
give rise to parts, the consideration of which is beyond the scope of this 
book. In regard to the details of the formation of the individual parts, 
the following statements may be made: 



THE ORAL CAVITY. 



413 



The Buccal Cavity.— The early appearance of the entrance to the 
alimentary canal is found in the formation of an open cavity bounded by 
the primitive maxillary processes above and mandibular arch below. 

The cavity thus formed is the common buccal space, the upper 
portion being the respiratory or nasal section, while below is the true 
mouth. The cavity of the mouth, as such, does not exist until these two 
are completely separated by the palatal plates forming the future roof 
of the mouth. 
* Figure 327 shows a vertical transverse section through this common 



Nasal Cavity 



Dental Ridge. 
Upper Jaw 



Dental Ridge, 
Lower Jaw 




Nasal Cartilage 



Buccal Cavity 



Tongue 



Dental Ridge, 
Lower Jaw 



Fig. 327. — Vertical Transverse Section through Head of Human Embryo, about the 

Tenth Week. x 30. 



buccal cavity. At this early period the lateral walls and floor of the 
mouth are manifest by certain cellular elements, but the roof of the 
cavity, as already stated, is not complete until the palatal plates, now 
separated by the tongue, grow inward and unite at the median line. 

The Oral Cavity. — The Roof of the Mouth. — When these two 
processes which arise from the mesoblast unite at the median line, they 
establish a permanent horizontal septum, dividing this part of the stomo- 
deum into a respiratory or nasal section and an oral section, the mouth. 
The cells entering into this part of the fetal head at this time (eight to 
tenth week) are of three varieties, being connective-tissue cells, cartilage 



414 



HISTOLOGY. 



cells, and epithelial cells, the latter being distributed in a layer of varying 
thickness over those parts destined to become a part of the lining mem- 
brane of the mouth. 

In figure 328 (twelfth week) the superior maxillary processes are 
shown united and the permanent separation between the mouth and nasal 
cavity established. This embryonal bridge is for the most part made up 
of connective-tissue cells, about isolated bundles of which osteoblasts 
arrange themselves, resulting in the production of two intermembranous 
bony plates. * 



Nasal Cavity 



Dental Ridge 



Tongu 




Cartilagenous Septum 
of Nose 



Den al Ridge 
Oral Cavity 



Meckel's Cartilage 



// 



Dental Ridge 



P 



V 

Anlageof Lowei Jaw 



Fig. 328. — Vertical Transverse Section through Head of Human Embryo, about the 
Twelfth Week, showing the Single Buccal Cavity Transformed into the Oral and Nasal 
Cavities. X 30. 



By the fourteenth week a further advance in the generation of the 
hard palate is noted, the septum now being largely composed of calcified 
tissue. The disposition for these primitive bony plates to exist as separate 
and distinct processes is exemplified at the median line by a definite 
separation formed by the connective-tissue sheath from which they are 
derived. Covering the surface of the hard palate there now appears a 
thin layer of mucous membrane. 

The Floor of the Mouth. — Having thus briefly noted the evolution 
of the roof of the mouth, let us next consider the floor of the cavity, the 
tongue and its attached muscles, together with considerable glandular 



THE FLOOR OF THE MOUTH. 



415 



tissue making up the bulk of this district. Figure 329 is a vertical 
transverse section through the floor of the mouth about the tenth week 
in the human fetus, or at a period somewhat later than that shown in the 
previous illustration. An examination of the parts in general at a time 
prior to this is of little value, save the early preparation for the develop- 
ment of the teeth, which will be referred to later. The tissues and 




Fig. 329. — Section through Base of Tongue and Lower Jaw. X 40. 



organs here shown will be recognized as the tongue (A), the glandular 
tissues (B), the forming jaw (C), with developing tooth-germs at D D. 

The tongue appears on the floor of the mouth between the thirtieth 
and thirty-sixth days as a bud from the mesoblast covered by a layer of 
cells of epiblastic origin. The muscle-fibers, be they intrinsic or ex- 
trinsic, are all of the striated variety. In a very short time, and at a 
comparatively early period, the tongue becomes an independent organ, 
presenting most of the characteristics common to it after birth. Not a 
small portion of the floor of the mouth is made up of another class of 



4i6 



HISTOLOGY. 



tissue which, although eventually a distinct organism, is composed almost 
entirely of epithelium. These cells, together with the connective-tissue 
cells, and eventually blood-vessels, unite in the production of a true 
salivary gland, the sublingual. Figure 330 shows the early character of 
the tissue, together with its relation to surrounding parts. The section 
is one from the region of the premolars, and is bounded above by the 
tongue, laterally by the borders of the jaw, and below by libers of the 




Fig. 330. — Section of Sublingual District. X ioo. 



mylohyoid and digastric muscles. Three distinct lobes or sections of 
the gland are observed, the two largest being separated by a reticular 
network of connective tissue. 

The general character of these developing glands even at this early 
period (about the twelfth week) appears to be very similar to the matured 
organ, being composed of a number of small tubes emptying into a single 
duct, constituting a gland of the compound tubular variety. 

Let us next give some consideration to the embryology of the mouth 



THE FLOOR OF THE MOUTH. 417 

in its entirety; and to do this, it is necessary to make sections of the parts 
in various directions. 

The growth of the cavity is usually studied, and probably to the 
best advantage, by vertical transverse sections, and attention will first be 
called to a number of sections made in this way, beginning at the lips and 
passing backward through the incisor region, and finally through the 
districts occupied respectively by the cuspids and molars. The period 



' 




1 


k^y 




^f 









Fig. 331. — Embryonal Labial Mucous Membrane. 

at which such an investigation is made has much to do with the character 
of the tissue involved, but the time best suited to the purpose is included 
between the fortieth and sixtieth days. At this time nearly all the tissues 
making up the organs and parts which enter into the construction of the 
cavity have advanced to such a degree of perfection that the investigation 
may proceed with considerable satisfaction. 

Figure 331 shows a cross-section through one of the primitive labial 
folds about the period named. Little is to be observed in this district 



4i8 



HISTOLOGY. 



at this early period except the simple cells of three varieties which serve 
to make up the parts, but attention is at once attracted to the abundant 
thickness of the epithelium given to the lip. 

If a section be made somewhat to the distal of that previously shown, 
a marked change in the relationship existing between the various cell 
layers is observed in a body of cells of another character, those which are 




Fig. 332. — Section through Base of Jaw. 

destined to become the cartilage of Meckel, and about which the younger 
layer of cells of mucous membrane are observed outlining a new district. 
If a section be made through this same location, say about the 
forty-eighth day, a vast change in the appearance of the parts is noticed 
(Fig. 327). The buccal walls of the mouth have in a measure become 
complete by a union of the upper and lower sections, the union at this 
time being accomplished through the agency of the embryonal epithelium. 
A cartilaginous nasal septum has made its appearance, and active prepa- 
ration for the ossification of the maxilke is apparent. In the center of the 



THE FLOOR OF THE MOUTH. 419 

section is a distinct body of cells forming Meckel's cartilage, and early 
preparations for the growth of the teeth may be seen at a by a dipping 
down of the surface epithelium. 

A transverse section through the same district about the sixtieth 
day (Fig. 333) shows all the parts strongly differentiated. Ossification 
has taken place to a considerable extent in the lower jaw, the two halves 
being at this period, and for some months afterward, separate and distinct. 




Fig. 333. — Section through the Wall of the Mouth of an Embryo, Sixtieth Day. 

Many muscle bundles are observed beneath the jaw, and beyond these 
the integument with its numerous blood-vessels and nerves, most of which 
are seen in transverse section. A cross-section upon the same subject 
about the sixtieth day in the region of the cuspids finds the tissues and 
organs advanced to a certain degree of perfection. (See Fig. 329.) The 
tooth-germs of the cuspid teeth have their crowns outlined by the cells 
composing them; the tongue with its complex muscular arrangement 
has become a specialized and independent organ, while beneath it we see 



420 



HISTOLOGY. 



that product of the epiblast, the glandular structure, so plentifully supplied 
to the floor of the mouth in this locality. 

Passing further back into the region of the molars, the appearance 
of the parts does not differ to any marked degree from that in the cuspid 
district, except in the general distribution of the muscular fibers of the 




Fig. 334. — Longitudinal Section through Chin of Embryo Lamb. 



tongue, and the appearance of the submaxillary gland, here appearing in 
three distinct lobes or parts. 

It has been previously stated that sections made in the direction of 
those already considered are usually employed to study these parts, but 
much is to be gained by supplementing these with sections made in other 
directions. 



Meckel's cartilage. 



421 



Figure 334 shows a longitudinal section, or one made from mesial to 
distal through the lower jaw at or near the median line, the parts included 
within the field being the labial folds at A, the mandible at C, the tongue 
at D, and a tooth-germ at E. An examination of the lips shows them to be 
covered with a varying thickness of embryonal epithelial cells which are 
continued backward over the future alveolar ridge and thence to the 
hard palate above, or over the floor of the mouth and the surface of the 
tongue below. 

Meckel's Cartilage. — One of the earliest products of the mesoblast 
is that which results in the production of Meckel's cartilage, which is 




Fig. 335. — Diagram to Illustrate the Metamorphosis of the Visceral 

Arches during Development. 

I, Presents the first visceral arch, it gives rise to Meckel's cartilage, which 

becomes transformed into the mandible. (After Wiedersheim.) 

closely associated with the growth and early support of the lower jaw. 
In the beginning, as already pointed out, the mandibular and hyoid arches 
resemble one another, but soon after they become fully established they 
take on different functions, and with this become dissimilar. The first 
appearance of this cartilage as a distinct body of cells is found about 
the middle of the second month, and when a transverse section of the 
jaw is made for the purpose of studying its location and environments 
(see Fig. 336), it is found near the base of the fetal head, considerably 
below and to the outside of the base of the tongue. At mid-jaw it appears 
as a circular body of cells separated from the surrounding parts by a 
distinct layer of elongated cells. Even at this early period a portion of the 
bony structure of the jaw is outlined by an aggregation of connective- 
tissue cells, and the forming cartilage appears to subserve the purpose of 



422 



HISTOLOGY. 



controlling the outline of the future jaw. The bow-shape of the cartilage 
is manifest as we pass toward the symphysis by the lateral halves approach- 
ing each other (Fig. 337), but the circular character of the cartilage in 
cross-section is still retained. 

Figure 338 represents a section through the symphysis about the 
eighteenth week, and shows the two halves of the cartilage closely asso- 




Fig. 336. — Section through Base of Lower Jaw, showing Meckel's Cartilage. 

ciated, but not united, the separation being by a layer of connective- 
tissue cells passing between the two. It will be noted also that the 
cartilage, instead of being near the base of the jaw as in figure 337, now 
appears near the floor of the mouth. 

Figure 339 shows the relations existing between the two halves of 
Meckel's cartilage and the growing mandible at the median line (a). 



Meckel's cartilage. 



423 



It also illustrates how little the development of the bone is dependent upon 
the cartilage, the growth of the former being in this district far below 
and apparently distinct from the latter. Here, as in the upper jaw, the 
periosteal cells from either side are observed to unite at the symphysis and 
pass as a somewhat thickened layer between the two bones, the only 
difference in the final change which takes place between the two being 
that in the upper. jaw a suture results, while in the lower jaw a layer of 




Fig. 337. — Section through Lower Jaw. M. C. Meckel's cartilage. 



solid bone is formed. The character of this cartilaginous framework as 
well as the cells which divide the two halves is shown in figure 340, the 
cartilage cells being oblong or cylindrical, with a bountiful supply of 
intercellular substance, while the connective-tissue cells are oblong or 
spindle-shaped. 

As soon as ossification in the jaw takes place to any extent, the 
cartilage begins to atrophy, that portion lying next to the jaw degenerating 



424 



HISTOLOGY. 




Fig. 338. — Meckel's Cartilage (M.C.) at the Symphysis. 



Meckel's cartilage. 



425 



first, so that by the tenth or twelfth week it has entirely disappeared, but 
before this takes place we find it surrounded by the periosteum, and 
finally completely inclosed within the bone. 

Figures 340 and 341 show the character of the cartilage cells about the 




Fig. 339. — Ossification of the Mandible at the Median Line. 



time that they are beginning to atrophy. It will be observed that the cells 
are inclined to a change in form, and that they are proportionately larger 
with large nuclei and nucleoli. A represents the district nearest the jaw, 
and the cells in this region have already lost their characteristic outline. 



426 



HISTOLOGY. 




Fig. 340. — Section of Meckel's Cartilage at Median Line. 



Meckel's cartilage. 



427 




Fig. 341. — Cartilage cells in the beginning of atrophy. 



428 



HISTOLOGY. 



FURTHER CONSIDERATION OF TOOTH 
DEVELOPMENT. 

One phase of the subject to which special attention will be given in 
this chapter is that which denotes the period at which the various events 
take place. This is a part of the study which is very difficult to deter- 
mine, and it would appear, for this reason, if for no other, that investiga- 




FlG. 342. — Transverse Section through Primitive Jaws of Human Fetus. 

tors of recent years have been perfectly satisfied to accept the results 
arrived at by their predecessors without any apparent effort to qualify the 
deductions. It has for a long time been conceded that the primitive 
changes which ultimately result in the formation of a tooth-germ are 
first noted in a heaping up of the epithelial cells over the district repre- 



TOOTH DEVELOPMENT. 



429 



senting the surface of the future jaw. While in many instances this is 
true, there are reasons why it cannot be considered an essential feature. 
In the first place, such a condition is not always present, as shown in fig- 
ure 342, a transverse section through the primitive jaw of a human fetus 
about the fortieth day. The tooth bands at A and B have penetrated 
the submucous tissue for a considerable depth, but the surface epithelium 
does not show a greater thickness at these points than it does over the 
general surface of the cavity. 



Ct 




73. 



Fig. 343. — Transverse Section through Lower Jaw, Human Embryo, Sixtieth Day. 



Figure 343 shows a section made in the same direction upon a human 
embryo about the sixtieth day, and while the tooth band (A) has pene- 
trated the embryonal connective tissue to a greater depth, there is yet no 
increase in the thickness of the epithelium, but rather a disposition for 
the parts to become depressed. Another reason why the heaping up of 
the superficial layer of cells forming the embryonal mucous membrane 
should not be considered the first sign of the preparation for tooth develop- 



430 HISTOLOGY. 

ment lies in the fact that these cells are not directly interested in the 
process, but that the inflection of cells which results in the formation of 
the tooth band results from the deep or infant layer of cells known as 
Malpighi's layer, as shown at B (Fig. 343). 

There is no question but that the location from which the section is 
taken has much to do with the character and thickness of the older layer 
of epithelial cells, and that they do at certain points constitute an epithe- 




FlG. 344. — Section through Tooth Band of Human Embryo. 

Hum exceeding in thickness that of other parts of the cavity, but this 
condition most frequently occurs after the enamel organ has assumed 
definite proportions. 

After the formation of the tooth band, which, it must be remembered, 
encircles the entire jaw in the form of a well-defined body of oval epithelial 
cells from the infant layer (shown at A in cross-section, Fig. 344), the 
next step in the process is one which concerns the location for the individ- 
ual buds for the enamel organs of the various teeth, and the approximate 
time at which these appear. 



TOOTH DEVELOPMENT. 



43 1 



In the human subject we find ten such spots appearing upon the 
lamina given off from the lingual face of the tooth band. These do not 
appear, however, at the free extremity of the band, but at some little 
distance toward the surface from this point, as shown in figure 345. In 
this section the tooth-germ is severed from the surface epithelium, but 
this is not a true condition at this period, as it still retains its connection 
with the surface by a narrow band of cells, the neck of the enamel organ. 




Fig. 345.- 



-Tooth-germ, Embryo Lamb, Corresponding to Sixtieth Day (Human). 



X 40. 



Two distinct classes of cells are now (sixtieth day) concerned in the 
process of tooth development, those at A being of epithelial origin and 
forming the future enamel organ, while at B an aggregation of cells from 
the mesoblast provides for the generation of the pulp and dentin. At C 
the narrow band of cells which should continue to the surface is shown, 
while the free extremity of the same body of cells at D will persist and 
eventually become the germ for the succeeding tooth. In regard to the 
time at which the buds for the various teeth appear, it might be expected 



43 2 



HISTOLOGY. 



that the same variation which follows the development and eruption of 
the teeth throughout would obtain, but such is not the case, the buds for 
the deciduous incisors appearing about the sixtieth day, while the germs 
for their permanent successors are but little later in forming. 

By referring to figure 334, which shows a longitudinal section through 
the lower jaw about the twelfth week, the deciduous incisor is seen well 
outlined by its formative cells, while immediately to the lingual appears a 




Fig. 345. — Tooth-germ, Premolar, Embryo Lamb. X 40. 

section of the germ for the permanent cuspid. Notwithstanding this, 
there elapses a period of several years between the eruption of these teeth. 
The same relative progress will be noted between the first and second 
teeth, be the subject human or otherwise; requiring many years to com- 
plete dentition, in the former, while in most of the lower animals the same 
process occupies but a comparatively short time. 

The next stage in the development of a tooth to which attention will 
be called is that in which the entire tooth-crown is outlined by the dentin 



TOOTH DEVELOPMENT. 



433 



papilla and surrounded by its epithelial cap, the enamel organ. Such 
an advance in the process is shown in figure 346, together with the sur- 
rounding structure. When this stage is reached, the individual cells 
of the tooth-germ are strongly differentiated, and the odontoblasts are 
making their appearance about the summits of the cusps. 




Fig. 347. — Section through Jaw of Embryo Lamb, in a District not Occupied by 

a Tooth-germ. X 40. 



Up to this time the cells present are those which result in the forma- 
tion of but two of the calcified tooth tissues, but now there is a marked 
disposition upon the part of the periosteum of the jaw to pass down by 
the side of the enamel organ (A), this being the first indication of 
the formation of the tooth follicle, the alveolodental membrane, and 
cementum. At this period it will be observed that there appears in this, 
28 



434 



HISTOLOGY. 



the molar region, a "heaping up" not only of the surface epithelium, but 
also of the connective tissue as well. 

Figure 347 shows a section through the growing mandible taken 
from a portion of the tissue not occupied by a tooth-germ. This is of 
interest, first, as giving a view of the detached tooth band in cross-section, 




FlG. 348. — Primitive Bud for Enamel Organ of Permanent Tooth, Human 

Embryo, x 300. 

at A; second, by showing the distribution of the periosteum to the interior 
of the jaw to serve the double function of the future tooth-sac and alveolo- 
dental membrane; and, third, the thickened epithelium with the underly- 
ing tissue pushing into it. 

Although the germ for the second tooth may be observed at a period 
somewhat prior to this, a study of some of its characteristics is best made 



TOOTH DEVELOPMENT. 



435 



at this time. The fact has already been referred to that this interesting 
phenomenon occurs soon after or even simultaneously with that for the 
first tooth, a portion of the primitive cord for the latter persisting as the 
germ for the former. 

In figure 348 the cells forming the primitive germ for the enamel 
organ of one of the permanent teeth are shown highly magnified. It 
will be observed that they are of the simplest epithelial character, and 
that they are derived directly from the enamel organ of the pre-existing 
tooth, on the one hand, while, on the other, they communicate with the 
surface by a narrow band of cells. In this way it is for a time dependent 
upon both of these parts for continuance and growth, but after a time 
it, too, like its predecessor, severs its connection with the surface, but 
remains intact with the epithelial cells of the former enamel organ until 
these cells begin to atrophy. 




Fig. 349. — Diagram of Various Forms of Hare-lip following the Lack of Union of Various 
Processes Participating in the Formation of the Mouth. 



The cells which made up this primitive germ are of three varieties: 
the inner layer, or those derived from the epithelium of the enamel organ 
of the first tooth, being small and spheroidal; those of the outer layer, 
which spring from the surface epithelium, being proportionately larger 
and cylindrical or oblong; while those which intervene are markedly 
irregular in outline. In this respect— that is, in the character of the early 
cell layers — the tooth-germs for the permanent teeth differ from those 
of the deciduous. 

The question of the origin of those teeth which have no predecessors 
is one upon which there has always been more or less discussion, some 
writers contending that they are derived directly from the oral epithelium 
by a special generation of cells for each tooth, while others are of the 
opinion that as the jaw grows backward certain changes take place which 
result in the establishment of an epithelial fold or lamina, in every partic- 



436 HISTOLOGY. 

ular corresponding to the tooth band of the deciduous teeth. With these 
two conflicting opinions in mind, a number of sections were made through 
the extreme distal end of the jaw. The result favored the latter theory, 
for here the tooth band is seen similar in form and location to that ob- 
served in the jaw in those locations from which succedaneous teeth result. 



CHAPTER VIII. 

Anomalies of the Teeth. 

It has always been conceded that the dental organs of man are sus- 
ceptible of much variation in form and structural arrangement, and 
that frequently this variation is so positive that the organ is pronounced 
anomalous in character. Just where the line of distinction between the 
normal and abnormal should be drawn is a subject worthy of some con- 
sideration. Some authorities define the word anomaly as a marked 
deviation from the normal, while, in the opinion of others, a much broader 
meaning is accorded it; and we find all those conditions which are in 
themselves an irregularity from the typical structure or occurrence included 




Fig. 350. — Anomalous Teeth. 

in this category. Under the first definition a given structure or organ 
is accorded a wide field for its normal existence, while under the latter 
but slight deviation is necessary to classify it among the abnormal. 

Upon first thought it would appear that the ability or inability of a 
tissue or an organ to perform its special function should, in a measure, 
decide the question of the nature of its being, and no doubt to a certain 
extent this is true; but while the action of an organ or a part of the body 
may, by observation, appear entirely satisfactory, it is only so at the 
expense of other organs or tissues, and these in the course of time, by this 
extra exertion, become hypertrophied or in other ways pathologic. 

While this is especially applicable to those organs or tissues which 
have a wide range of function, it may with a good deal of force be applied 

437 



438 ANOMALIES. 

to the dental organs and their immediate environments. Anomalous 
conditions in the teeth may originate in, or be confined to, one or more 
of the tooth tissues, in any of which the structural disarrangement may 
eventually result in the death or degeneracy of the part. Enamel mal- 
formation is of such a character that it may be observed upon the surface 
either in the form of a multiplication of cusps, or by an extra development 
of the various ridges formed by pronounced folds of this tissue. But 
probably the most disastrous anomaly of the enamel, and one frequently 
responsible for the downfall of this tissue, is found in some defect of its 
structural arrangement other than those just referred to. In some 
instances the enamel rods of a given district, instead of being normally 
distributed by assuming a direction principally at right angles to the long 
axis of the tooth crown, are arranged without regard to the base or per- 
iphery of the tissue, and we have as a result an anomaly of structure. 
The question of normal and abnormal rod distribution now presents 
itself, because in certain locations — i.e., the summits of the cusps — 
an arrangement of the rods similar to that referred to is so common that 
it may be considered a normal condition, while, if a like distribution was 
found in other locations, the tissues should properly be considered 
abnormal. 

Malformed teeth, in respect to the number and forms of the 
cusps present, are not alone confined to the enamel, but also to the dentin 
which first records the tooth form on its periphery. 

Anomalies in the general contour of the tooth crown are usually 
confined to the incisors and third molars, both the dentin and the enamel 
contributing to the deformity. Here the defect is usually so pronounced 
that but little difficulty is experienced in properly classifying the organ. 
One of the most frequent variations in form met with in these locations 
is found in the peg-shaped or cone-shaped crown. If it were possible 
it would be interesting to trace the development of such a malformation; 
but with our present knowledge of this process in general, there is little 
doubt as to its origin, the enamel organ failing to fulfill its early and pri- 
mary function of moulding the tooth crown in the dentin papilla, the 
responsibility for this resting in the special cells composing it, as well as 
the so-called stellate reticulum, which, it is believed, exerts a controlling 
influence over the form of the enamel cap. 

While the organic defects of tooth crowns are numerous and varied, 
those which are confined to the roots are most frequent, in many instances 
interfering to some extent with the function of the organ. When a given 
peculiarity is confined to this portion of the tooth, it is frequently difficult 



SUPERNUMERARY TEETH. 439 

to discriminate between the normal and the abnormal. Certain teeth 
are recognized as normal when either a single root or two roots are pres- 
ent, and the acceptance of this fact increases the difficulty of a proper 
classification of its peculiarities. 

In very rare instances do we find the roots of the cuspidate teeth 
more or less crooked; yet, at the same time, many decidedly crooked 
roots are considered within the natural law; while, on the other hand, 
roots with but little more deflection are classed as anomalous. 

Marked flexions of roots or crowns, cases of fusion or concrescence, 
are usually so positive in character that an anomalous condition is at 
once acknowledged. While tooth anomalies are usually referred to as 
external, or as belonging to the hard tissues of the organ, they are not 
infrequently found in the pulp or pulp cavity. This cavity, normally 
following the external contour of the tooth, is subject to much variation 
in outline and capacity, regardless of those changes which are incident to 
the continuous process of dentinification. 

Among these are a complete division of the pulp chamber; horn- 
like processes penetrating the dentin in the direction of the occlusal sur- 
face in locations where they would be least expected; an unusual number, 
or a peculiar distribution of the canals, etc. 

Lack of Dentition. — Cases in which there is a total absence of 
teeth have been reported. Guilford reports the case of a man fifty years 
of age who never had teeth, the jaws not differing in appearance or form 
from those of a person whose teeth have been extracted. The mother 
of the subject had the usual number of teeth, but the grandmother and 
an uncle were both edentulous and hairless from birth. J. Tomes 
mentions two cases having been reported to him, and Linderer mentions 
one. 

On the other hand, instances of a third dentition have been reported, 
but there is great possibility of such observation being erroneous. Teeth 
belonging to the permanent set which may have remained unerupted 
for years could readily be mistaken as predecessors of a third set, when 
in after years they made their appearance. 

Supernumerary Teeth. — All teeth appearing in the mouth in addition 
to the normal number are designated as supernumerary teeth. These are 
divided into two classes, those normal in size and form and those abnormal 
in size and form. The first-named are most likely to be of the simple 
class, incisors and cuspids, and they may occupy a regular position in 
the arch or may be found inside the arch closely associated with teeth of 
the same type. Supernumerary bicuspids and molars are sometimes 



44° ANOMALIES. 

present either in regular position in the arch or inside of it. When the 
jaws are long enough to accommodate them, an additional molar may 
appear back of the third molar, making four molars instead of three, 
and cases in which two extra molars were thus placed have occasionally 
been met with. 

Supernumerary teeth of the second class or those which are abnormal 
in size and form are usually inclined to be cone-shaped and small in size, 
this in respect to the root as well as the crown. Supernumerary teeth 
of this character are usually found in the incisor region, but it occasionally 
happens that they are found in the molar district, but here, instead of 
having a single cone for the crown, they are mostly made up of a number 
of smaller cones resembling many small cusps on the occlusal surface. 
The number of supernumerary teeth may vary from one to eight or ten. 
In the latter instance they are usually scattered through the entire alveo- 
lar border of the hard palate. As many as ten or twelve teeth thus 
located have been reported. 

Again, certain teeth are frequently missing from the arch. This 
may be occasioned by delayed eruption, or it may be the result of im- 
proper activity within the tooth-germ itself, so that the tooth may have 
failed to develop. The teeth most frequently missing are the upper 
lateral incisors. The probable reason for this lies in the fact that the 
germs for these teeth are located very near the surface of the bone, and 
in some instances they are not even protected by a thin layer of bone 
over the follicle. Being thus situated, they are more or less exposed to 
violence sufficient to destroy the germs and thus render development of 
the teeth impossible. Lack of certain teeth in the mouth appears to be 
to some extent an hereditary feature, the condition being transmitted 
from parent to child. Anomalies as to the size of individual teeth are 
frequently noticed. When this is the case, it does not usually include 
the entire set, but is confined to one or two, usually to the same teeth on 
each side. Teeth that are above the usual size are generally found in 
persons of large build, but if all the teeth are proportionate in size they 
cannot be included within the abnormal class. The upper incisors 
are most frequently affected in this way. Cases have been reported in 
which the central incisors in the upper jaw have been fully twice the size 
which they should normally have been, and all the remaining teeth in 
the mouth perfect as to shape and size. Accompanying the abnormal 
condition it is usual to find the teeth thus affected more or less abnormal 
in outline, but, notwithstanding this, retaining their form sufficiently 
well to permit their proper classification. 



SUPERNUMERARY TEETH. 



441 



There are likewise teeth that are deficient in size. This does not 
refer to teeth all relatively small, as frequently found in persons of small 
frame and stature. This anomalous condition is often present in the 
upper lateral incisors and in the third molars. Teeth that are deficient 
in size generally possess their normal shape. 

While the crowns of the teeth are susceptible to the above variations, 
the roots appear to be anomalous much more frequently and to a more 
marked extent than are the crowns. Most important among these may 
be mentioned flexions of the roots. Curvatures in the roots may be 
found either in single-rooted teeth or in multi-rooted teeth, and the point 
of flexion may be located either at the center of the root or near its apex, 




Fig. 357. — Anomalous Roots. 

or both of these points may be affected. Flexions in the roots of the 
teeth present a great variety in form. A single curve in one direction 
may be present or a number of curvatures in different directions. In 
multi-rooted teeth the roots may be so flexed that they will entwine about 
each other, and the overlapping portions may be or may not be fused. 
Probably the most important cause for the flexions of the roots of the 
teeth is delayed eruption, this being particularly true if some positive 
force prevents the progress of the organ. The roots of the teeth are 
frequently anomalous in regard to number. These may have the same 
general form and the same approximate length as the normal root or 
roots would be, but they are proportionately smaller in diameter. Cases 
are on record in which the incisor teeth have had two distinct roots, but 
probably the most frequent location for the multiplicity of roots is found 
in connection with the third molar. While this tooth when normally 
developed as to crown is usually supported by three roots, it sometimes 
possesses five or six smaller roots. 



44-' 



ANOMALIES. 



It not infrequently happens that the roots of the teeth are less in 
number than they should normally be. This is usually brought about 
by the blending of the roots, which occurs during the developmental 
stage. This may be of two distinct kind c . In the first place, it may 
be the result of the conversion of two or more pulp canals into one, or the 
individual roots may be united by a body of cementum being interposed 
between them. In the former instance a single pulp canal is mostly 
found within the blended roots, while in the latter the number of canals 
is usually normal. With this blending there is generally a line of demar- 
cation between individual roots as they should normally exist, in the 




Fie;. 352. — Fusion of Molars. 



way of longitudinal depressions extending through the entire root from 
cervical line to apex. This anomaly, like that of the multiplicity of roots, 
is principally confined to the third molars, and is more frequent in the 
upper jaw than in the lower. 

Fusion and Concrescence. — The union of two or more teeth is 
known as fusion or concrescence, and may occur either during the develop- 
ment of the organ or after this process has been completed. When union 
takes place during development, it is characterized as fusion; when it takes 
place after the completion of this process, it is known as concrescence. 
There seems to be but little doubt that fusion of the teeth occurs through 
some irregularity in the tooth-germ or germs, the beginning of the develop- 
mental process taking place generally between two germs and continuing 
together until the complete calcification of the organs. Teeth thus 
united may have an internal anatomy corresponding in nearly every 
respect to two separate teeth, and, on the other hand, they may possess 



FUSION AND CONCRESCENCE. 



443 



but a single pulp chamber and canal. Fusion may take place in the 
roots of the teeth alone, when it is called partial fusion; but when it is 
confined to the roots and crowns alike, it is classified as complete fusion. 
The teeth most likely to be affected in this way are the upper incisors 
and the second and third molars. Some distinction must be made be- 
tween those teeth which are united by a layer of cementum, and those of 




Fig. 353. — Fusion and Concrescence. 




Fig. 354. — Fusion and Concrescence. 



true fusion, the latter existing only when there has been a union between 
dentin and/ dentin. 

Concrescence. — In concrescence, the roots of the teeth only can be 
affected, as the union takes place after the complete development of the 
organ. The roots of one tooth become united to the roots of another 
through an additional growth of cementum, this growth being sufficiently 



444 



ANOMALIES. 



extensive to cause absorption of the alveolar septa by pressure. Follow- 
ing this there is a resorption of the pericementum. The cemental tissues 
of the teeth are then brought in contact, and coalescence gradually takes 
place. Concrescence may take place not only between the roots of two 
teeth, but between the various roots of an individual tooth, this resulting 
in the same manner as above described by the destruction of the septa 
within the tooth socket. When concrescence takes place, the process 
is usually confined to the apices of the roots, although they may become 
coalesced throughout their entire length. The teeth most commonly 
affected by concrescence are the molars and bicuspids, from the fact 
that the alveolar walls, particularly the septa about these teeth are espe- 
cially thin, owing to the form and relative location of the roots. 




Fig. 355. — Geminous Tooth. 



Geminous Teeth (Fig. 355). — It occasionally happens that two 
separate germs are confined within a single sacculus, and from this 
results two teeth, either similar or -dissimilar in size and form. One of 
the pair may be normal as to form and size, while the other may be much 
below the normal size, but more or less perfect in outline. Geminous 
or twin teeth may be united or entirely separate. This condition is 
most frequently found in the molar teeth, although cases in which the 
bicuspids and incisors have been thus affected are recorded. Teeth 
thus formed must not be confounded with those in which fusion is the 
anomaly. In geminous teeth a single sac contains two tooth-germs 
from which result two similarly formed teeth; in fusion two follicles 
coalesce, each of which contains its own germ. 

Besides the foregoing, the roots of the teeth are subject to anomalies 
in size, in some instances being abnormally small, in others abnormally 
large. In the former they may nearly always be characterized as anoma- 



GEMINOUS TEETH. 



445 



lous, but this is not always true of the latter. When the roots are abnor- 
mally large, it is somewhat difficult to discriminate between an anomalous 
and a pathologic condition, the latter usually being the case when the 




t FiG. 356. 




Fig. 357- 

roots of individual teeth are affected. Roots, to be considered anoma- 
lous in size, should in a measure retain their normal form, cases of hyper- 
trophy (hypercementosis) usually resulting in the destruction of the 
normal contours (Figs. 356 and 357). 



INDEX. 



Accessory palatal foramina, 18 
Achromatin substance, 298 
Achromatic spindle, 301 
Adipose tissue, 349 
Alveolar process, 41 

development of, 45 
Alveoli, 42 
Alveolodental membrane, 73, 102, 403 

blood supply to, 104 

cells of, 404 

fibers of, 406 

histology of, 403 

interfibrous elements of, 409 

nerve supply to, 104 
Amelification, 361 
Ameloblasts, 243, 360, 362 
Anatomy, macroscopic, 293 

microscopic, 293 
Angular artery, 13 
Anomalies of the teeth, 437 
Anterior palatal foramen, 18 

nerve, 18 

superior dental nerve, 97 
Antrum of Highmore, 45 
Apical foramina, description of, 198 
Areolar tissue, 348 
Artery, deep facial, 18, 32 

inferior dental, 93 

internal maxillary, 18, 115 

lingual, 32 

ranine, 32 

sublingual, 32 
Azygos uvulae, 23 

Bicuspid, lower first, 180 
second, 183 
upper first, 133 
second, 143 
Blastoderm. 307 

layers of, 307 
Blood and lymph, 294, 326 
development, 326 
physiology, 327 
plasm, 326 
structure, 327 
course of, from heart to cheeks, 15 
from heart to hard palate, 18 
from heart to lips, 8 
from heart to soft palate, 23 
from heart to tongue, t,^ 
supply to the teeth, 91 
Bone, 315 

canaliculi of, 316 
cells of, 316 



Bone, Haversian canals of, 316 

histologic examination of, 316 

hyoid, 25, 59 

inferior maxillary, 51 

lacunae of, 316 

marrow of, 318 

matrix of, 315 

osteoblasts of, 318 

palate, 47 

periosteum of, 318 

Sharpey's fibers of, 317 

superior maxillary, 34 
Bones, of the mouth, 34 

superior maxillary, 34 
Branchial arches, 411 
Brown stria; of Retzius, 355 
Buccal cavity, 413 

embryology of, 412 

glands, 105, 341 

orifice, 2 
Buccinator muscle, 9 

Calcification, beginning of, 268 

of cementum, 282, 395, 439 

of dentin, 268, 377 

on enamel, 361 
Canal, inferior dental, 55 

infra-orbital, 35 

posterior palatal, 38 
Canaliculi, 316 
Canals, Haversian, 316 

pulp, 198 
Canine eminence, 35 

fossa, 35 
Capsular ligament, 64 
Cartilage, 313 

cells of, 314 

elastic, 314 

fibro-, 314 

hyaline, 314 

matrix of, 313 

Meckel's, 57, 261, 421 

permanent, 315 

temporary, 315 

varieties of, 313 
Cell, amitosis, 303 

conductivity, 300 

definition, 296 

division, 302 

fat, 313 

g'and, 313 

growth, 299 

irritability, 299 

karyokinesis, ^02 



447 



44 8 



INDEX. 



Cell, membrane, 295, 298 

metabolism, 297, 299 

mitosis, 302 

motion, 300 

nucleus, 294, 297, 298 

origin, 296 

plasma, 313 

pigment, 313 

protoplasm, 294, 295, 297 

spontaneous generation, 296 

wall, 298 

vital manifestations, 299 ■ 
Cement corpuscles, 391 

fibers, 393 
Cementification, 282, 395 
Cementoblasts, 282, 405 
Cementum, 72, 385 

calcification of, 282, 395 

canaliculi of, 386 

fibers of, 393 

histology of, 385 

lacunas of, 386 

lamella?, 390 

matrix of, 386 
Central incisor, lower, 173 

upper, no 
Centrosome, 297 
Cervical line, 77 
Cheeks, 8 

blood supply to, 13 

external covering of, 9 

glands of, 9, 105, 341 

integument of, 9 

internal covering of, 9 

mucous membrane of, 9, 330 

muscles of, 9 

muscular tissue of, 348 

nerves of, 15 

substance of, 9 
Chromosomes, 302 
Chromatin substance, 298 
Circumvallate papilla;, 27 
Close skein, 302 
Ccelom, 311 

Concrescence of teeth, 487 
Condyle, neck of, 56 
Condyloid process, 56-63 

forms of, 63 
Connective tissue, cells of, 312 

classification of, 312 

fibrous, 312 

intercellular substance of, 312 

origin, 310 
Coronoid process, 56 
Crowns, anomalous, 485 
Cuspid, lower, 177 

upper, 124 
Cytoplasm, 297 



Decalcification, 290 

Deciduous lower central incisor, 
cuspid, 235 
first molar, 231, 236 
lateral incisor, 228, 234 



234 



Deciduous lateral incisor, measurement, 227 

second molar, 233, 238 

teeth, decalcification of, 227, 290 
detail description of, 227 
enamel organs for, 243 
general description of, 224 
occlusion of, 226 
pulp chambers and canals of, 238 

upper central incisor, 227 
cuspid, 229 
Deep facial artery, 18, 23 

branches of, 23 
Dendrites, 322 
Dental arch, 77 

arrangement of teeth in, 78 
curve described by, 82 
influence of temperament on, 83 

follicle, 242, 258 

formula, 75 
Dental furrow, 264 

pulp, 396 

sacculus, 258 
Dentin, 72, 368 

calcification of, 377 

cells, 257, 258 

chemical analyses of, 369 

exposed by dissection, 266 

fibers of, 368, 372 

granular layer, 376 

history of, 368 

matrix of, 369 

organ, 242, 256 

papillae, 257, 377 

tubules of, 368, 370 
Dentinal fibers, 372 

sheaths, 368, 371 

tubules, 368, 370 
walls of, 371 
Dentinification, 377 
Dentition, completion of, 290 

lack of, 439 
Depressor anguli oris, 12 

labii inferioris, 7 

labii superioris, 6 
Development of enamel, 358 

of permanent teeth, 276 

of teeth, 240, 470 
Deutoplasm, 298 
Diaster stage, 302 
Digastric fossa, 53 
Duct, parotid, 108 

sublingual, 109 

submaxillary, 108 



Ectoderm, 307 

Elastic cartilage, 314 

Embryology, general, 293, 300, 304 

of mouth and teeth, 411 
Enamel, 72, 350 

ameloblasts of, 360 

brown stria; of, 355 

calcification of, 361 

cells, 243, 361 

chemic composition of, 351 



INDEX. 



449 



Enamel, cuticle of, 401 
development of, 358 
histology of, 350 
organ, 242, 243, 398 
cells of, 243, 250 
external epithelium of, 243, 359 
form of, 243, 245 
internal epithelium of, 243, 359 
stellate reticulum of, 243, 359 
stratum intermedium of, 243, 359 
prisms of, 351, 362 
rods, formation of, 351 
Endomysium, 320 
Endothelium, 326 
Endoplasm, 298 
Entoderm, 307 
Epimysium, 320 
Epithelial cells, 309 

tissues, 294, 309 
Eruption of the teeth, 282 
External maxillary artery, 13 
oblique line, 52 
pterygoid, 70 
Exoplasm, 298 



Facial angle, 88 

artery, 15, 32 
Facial artery, branches of, 13 

nerve, 15 

branches of, 15 
Falciform papilla?, 27 
Fat- cells, 313 
Fauces, anterior pillars of, 20 

isthmus of, 20 

pillars of, 20 

posterior pillars of, 20 
Fibers, 297 

dentinal, 372 

Sharpey's 407 
Fibroblasts, 406 
Fibro-cartilage, 314 
Fibrous connective tissue, 312 
Fifth nerve, 96 

division of, 96 
Filiform papillae, 27 
Floor of the mouth, boundaries of, 25 

embryology of, 414 

framework of, 25 
Follicle, dental, 242, 258 
Foramen, anterior palatal, 18 

apical, description of, 198 

cascum, 27 

incisive, 41 

infra-orbital, 35, 36 

mental, 53 
Foramina, accessory palatal, 18 

posterior palatal, 18 
Fossa, glenoid, 62 
Frenas of the mouth, 100 
Fungiform papillae, 27 
Fusion of teeth, 442 

and concrescence, 442 

complete, 443 

partial, 443 
29 



Ganglion, Gasserian, 95 
Meckel's 19 
sphcnopalatal, 19 
Geminous teeth, 444 
Genial tubercles, 53 
Geniohyoglossus, 29 
Germinal cells, 304 

layers, 307 
Germs for permanent molars, 258 
Gills, 411 
Gingivae, 17 
Gingival border, 17 

margins, outlines of, 100 
Gland cells, 313 
follicles, 339 
parotid, 106, 344 
saccular, 339 
tubular, 339 
sublingual, 109, 345 
submaxillary, 108, 344 
Glands and ducts of the mouth, histology of, 
339 
buccal, 105, 341 

excretory ducts of, 340 
labial, 105, 338 
lingual, 106, 339 
molar, 105, 340 
of the cheeks, 341 
of the hard palate, 341 
of the mouth, 105, 339 
of the soft palate, 106, 341 
palatal, 106, 341 
salivary, 106, 343 
Glenoid fossa, 62 
Gomphosis, 73 
Groove, infra-orbital, 36 
Ground substance, 294 
Gubernaculum, 272 

foramina of, 278 
Gums, 99 

epithelium of, 333 
fibrous tissue of, ^^3 
general description of, 99 
mucous membrane of, 100, 332 



Hard palate, 16 

arch of, 17 

blood supply to, 18 

bones of, 17 

covering of, 18 

formation of, 18 

glands of, 17, 341 

mucous membrane of, 17, 334 

nerves of, 19 
Hare-lip, 435 
Haversian canals, 316 
Heredity, 302 
Highmore, antrum of, 45 
Histology, 293 
Histogenesis, 293, 307, 304 
Hooke, Robert, 294 
Hyaline cartilage, 350 
Hygolossus, 28 
Hyoid bone, 59 



45° 



INDEX. 



Hyoid development, 60 

greater cornua, 59 

lesser cornua, 59 

muscles attached to, 60 
Hypoblast, 457 
Hyaloplasm, 298 

Incisive foramen, 41 

fossa, 36, 52 
Incisor crest, 41 

lower central, 173 

lateral, 176 
upper central, no 
lateral, 118 
Inferior coronary artery, 8, 13 
vein, 13 
dental artery, 93 

incisive branch of, 95 
mental branch of, 95 
canal, 56 
meatus, 40 
nerve, 98 
lingualis, 32 
maxillary bone, 51 
body of, 52 
development of, 57 
facial surface of, 5 2 
internal surface of, 53 
muscles attached to, 57 
vertical portion of, 55 
palatal vein, 18 
turbinated crest, 37, 40 
Infra-orbital canal, 36 
foramen, 36 
groove, 36 
Intercellular substance, 311 
Interglobular spaces, 376 
Internal maxillary artery, 18, 91 

infra-orbital branch of the, 91 
superior maxillary branch of the, 91 
oblique line, 53 
pterygoid, 70 
[nterproximate spaces, 83 
Involuntary muscular tissue, 319 
Isthmus of fauces, 20 

Labial glands, 105, 340 
Lacrimal canal, 38 

tubercle, 38 
Lacuna?, 316 
Lateral incisor, upper, 113 

lower, 176 
Layer, granular, 376 

Malpighi's, 430 
Levator akeque nasi. 4 

anguli oris, n 

labii inferioris, 6 
superioris, 6 

palati, 22 
Lines of Schreger, 357 
Lingual artery, 32 

glands, 105, 379 

vein, 33 
Lips, 2 

blood supply to, 7 



Lips, external covering of, 3 
frenae of, 3, 100 
glands of, 3, 105, 340 
integument of, 3 
internal covering of, 3 
mucous membrane of, 3, 329 
muscles of, 3 
muscular tissue of, 347 
nerves of, 8 
substance of, 3 
Loose skein, 302 
Lower bicuspid, first, 180 

buccal surface of, 181 
calcification of, 180 
cusps of, 180 
distal surface of, 182 
lingual surface of, 182 
measurements of, 180 
mesial surface of, 182 
neck of, 182 
occlusal surface of, 180 
pulp cavity of, 221 
root of, 183 
second, 183 

buccal surface of, 184 
calcification of, 183 
distal surface of, 186 
lingual surface of, 185 
measurements of, 183 
mesial surface of, 185 
neck of, 186 

occlusal surface of, 183 
root of, 186 
bicuspids, general description of the, 180 

pulp cavities of, 221 
cuspid, 177 

calcification of, 177 
cusp of, 179 
distal surface of, 178 
labial surface of, 177 
lingual surface of, 178 
measurements of, 177 
mesial surface of, 178 
neck of, 179 
root of, 179 
cuspids, pulp cavities of, 220 
incisor, central, 173 

calcification of, 173 
cervical margin of, 175 
cutting-edge of, 175 
rlistal surface of, 17^ 
labial surface of, 173 
lingual surface of, 173 
mesial surface of, t 74 
measurements, 173 
neck of, 176 
root of, 176 
incisors, pulp cavities of, 219 
lateral incisor, general description of, 176 
molar, first, 186 

buccal surface of, 189 
calcification of, 186 
cusps of, 188 
distal surface of, 191 
lingual surface of, 190 



INDEX. 



451 



Lower molar, first, measurements of, 186 
mesial surface of, 190 
neck of, 191 
occlusal surface of, 186 
roots of, 191 
second, 192 

buccal surface of, 193 
calcification of, 192 
distal surface of, 195 
lingual surface of, 194 
measurements of, 192 
mesial surface of, 194 
occlusal surface of, 192 
roots of, 195 
third, calcification of, 195 

general description of the, 195 
roots of, 196 
types of the, 197 
molars, pulp cavities of, 222 

Malpighi's layer, 430 
Mandible, 51 

evolution of, 261 
Masseter, n, 89 
Mastication, muscles of, 68 

active organs of, 1 
Matrix, 311 
Maxillary bones, development of, 260 

sinus, 45 
superior, 34 
Meckel's cartilage, 59, 261, 421, 466 

ganglion, 19 
Median raphe, 27, 102 
Membrana eboris, 257, 398 

praformativa, 362 

propria, 313 
Mental foramen, 53 

protuberance, 52 
Mesoderm, 307 

parietal, 311 

visceral, 311 
Meraplasm, 298 
Middle meatus, 40 

superior dental nerve, 97 
Microsomes, 298 
Moral glands, 341 

lower, first, 186 
second, 192 
third, 195 

upper, first, 146 
second, 158 
third, 165 
Monaster stage, 302 
Morula, 305 
Motion, 300 

amoeboid, 300 

ciliary, 300 

contractile, 300 
Mouth, 1 

angles of, 2 

bones of, 34 

boundaries of, 1 

contents of, 1 

development, 411 

dissection of, 15 



Mouth, divisions of, 15 

entrance to, 1 

epithelium of, 328 

floor of, 16, 24 

frenoe of, 100 

general description of, 1 

glands of, 102, 339 

inferior portion of, 15 

interior of, 15 

lateral walls of, 8 

mucous membrane of, 101, 328 

muscular tissues of, 347 

posterior boundary of, 20 

primitive, 411 

roof of, 15, 16 

situation of, 1 

submucosa of, 329 

superior portion of, 15, 16 

tunica propria of, 329 

vestibule of, 1 

walls of, 8 
Mucous membrane of the checks, histology 

of, 33 x 
of gums, histology of, 332 
of lips, histology of, 329 
of mouth, 101, 328 
blood supply to, 329 
histology of, 329 
nerve supply to, 329 
of tongue, histology of, 335 
Muscle, external pterygoid, 70 
internal pterygoid, 70 
masseter, n, 68 
temporal, 69 
Muscles, angular series, 20 
of mastication, 68 
of soft palate, 20 
of tongue, 28 
Muscular tissue, 318 

tissues of cheek, 348 

endomvsium of, 356 
involuntary, 319 
of lips, 347 

perimysium of, 320 
sarcolemma of, 320 
sarcoplasm of, 320 
of mouth, histology of, 347 

non-striated, 319 
of soft palate, 348 

striated, 320 
of tongue, 349 
voluntary, 320 
Mylohyoid ridge, 53 
Myrtiform fossa, 36 

Nasal crest, 41 

spine, 41 
Nasmyth's membrane, 385, 401 
Nerve, anterior palatal, 19 

cell, 321 

corpuscles, 324 

fiber, axis cylinder of, 324 
neurilemma of, 324 

fibers, 324 

medullated, 325 



452 



INDEX. 



Nerve fibers, non-medullated, 325 

inferior dental, 98 
maxillary, 98 

middle superior dental, 97 

posterior superior dental, 97 

process, 323 

superior dental, 97 

superior maxillary, 96 
Nerves, 324 

endoneurium of, 325 

epineurium of, 325 

funiculi of, 324 

medullary sheath of, 325 

perineurium of, 322 

of tongue, 33 

system of, 322 
Nervous tissue, 321 
Neumann's sheath, 371 
Neurilemma, 325 
Neuroblasts, 322 
Neurocyte, 321 
Neuron, 321 

Non-striated muscular tissues, 319 
Nucleus, 294, 297, 298 
Nuclear membrane, 298, 307 

reticulum, 298 

stain, 296 
Nucleoplasm, 297 
Nucleolus, 299 

Occlusion of the teeth, 85 

of the deciduous teeth, 226 
Odontoblastic cells, 257 
Odontoblasts, 257, 379, 398 

processes of, 499 
Oral cavity, 411 

embryology of, 411 

plate, 411 

sinus, 41 1 
Orbicularis oris, 4 
Organogenesis, 293 
Organs and tissues, 303 
Osteoblasts, 354, 404 
Osteoclasts, 406 
Overbite, 88 
Ovum, 296, 304 

fertilization, 304 

maturation, 305 

segmentation, 305 

Palatal glands, 106, 341 
raphe, 16, 102 
ruga?, 16, 102 
Palate, bone, articulation of, 50 

attachment of muscles to, 50 
blood supply to, 50 
development of, 50 
horizontal plate of, 48 
vertical plate of, 48 
bones, 47 
hard, 15, 16 
soft, 19 
Palatoglossus, 20 
Palatomaxillary suture, 18 
Palatopharyngeus, 22 



238 



Papilkc of the tongue, 27 
Paraplasm, 298 
Parotid duct, 108 

gland, 106, 343, 382 
Perichondrium, 350 
Perimysium, 320 
Periosteum, 318 

Permanent incisors, papillae for, 278 
molars, germs for, 258 
teeth, 73 

preparation for development of, 276 
advance of, 290 
Phagocytes, 300 
Pigment cells, 313 
Pillars of fauces, 20 
Plasma cells, 313 
Posterior foramina, 18 
palatal canal, 38 
superior dental nerve, 96 
Primitive dental furrow, 264 
Process, alveolar, 41 
condyloid, 56 
coronoid, 56 
fronto-nasal, 411 
Protoplasm, 294, 297 
Protoplasmic stains, 296 
Pulp, blood-vessels of, 400 
canals, 198 

cavities, description of, 198 
dissections to show, 198 
horns of, 199 
of deciduous teeth, 
of lower teeth, 219 
of teeth, 198 
of upper teeth, 200 
cells of, 379 
histology of, 379 
nerves of, 400 
odontoblasts of, 397 

Quadratus menti, 7 

Ranine artery, 32 
Raphe, 16, 27, 102 
Retzius, brown slriaj of, 355 
Ridge, mylohyoid, 53 
Risorius muscle, 12 
Roots, anomalous, 485 
formation of, 282 
Ruga, 16, 102 

Sacculus, dental, 258, 307 
Salivary glands, 106, 343 

blood-vessels of, 345 

nerves of, 345 
Sarcolemma, 320 
Sarcoplasm, 320 
Schleiden, 294 
Schreger, lines of, 395 
Schultze, Max, 296 
Schwann, 295 

sheath of, 362 

white substance of, 362 
Seventh nerve, 15 
Sharpey's fibers, 407 



INDEX. 



453 



Sheath, medullary, 322 

of Schwann, 323 
Simple saccular glands, 339 

tubular glands, 339 
Soft palate, 19 

blood supply to, 23 
glands of, 341 
muscles of, 20 
muscular tissue of, 334, 348 
nerves of, 23 
substance of, 20 
Spaces, interglobular, 376 

interproximate, 83 
Spermatozoon, 304 
Sphenomaxillary ligament, 64 
Sphenopalatal ganglion, 19 
Spongioplasm, 298 
Stellate reticulum, 243, 359 
Stenson, foramen of, 41 
Stratum intermedium, 243 359 
Striated muscular tissue 356 
Styloglossus, 31 
Stylomaxillary ligament, 65 
Sublingual artery, 32 
duct, 109 
fossa, 53 
gland, 108 
Submaxillary duct, 108 
fossa, 53 
gland, 108 
Submucosa, 329 
Supernumerary teeth, 439 
Superior coronary artery, 8, 13 
vein, 13 
lingualis, 32 

maxillary bone, articulation of, 44 
alveolar process of, 41 
blood supply to, 44 
development of, 44 
facial surface of, 35 
malar process of, 40 
muscles attached to, 44 
nasal process of, 39 
orbital surface of, 34 
palatal process of, 37, 40 
proximal surface of, 37 
sinus of, 45 
tuberosity of, 39 
zygomatic surface of, 39 
bones, 34 
nerve, 96 
meatus, 40 
palatal vein, 18 
turbinated crest, 40 
Suture, palatomaxillary, 18 
Symphysis, 52 

Teeth, 72 

anterior, 76 

apical extremities of, 76 
articulation of, 85 
attachment of, 72 
blood supply to, 91-95 
classification of, 72, 73 
complex, 72 



Teeth, deciduous, 73, 224 

description in detail of, no 

development of, 240, 428, 470 

dissections of, 198 

division of, 7 2 

eruption of, 334 

names of, 75 

nerve supply to, 95 

occlusion of, 85 

permanent, 73 

posterior, 76 

pulp cavities of, 198 

roots of, 75 

simple, 72 

surfaces of, 75 

tissues of, 350 

veins from, 95 
Temporal muscle, 69 
Temporomandibular articulation, 61 

movements of, 66 
Tensor palati, 23 
Tissues of the body, divisions of, 294 

of the teeth, 350 
Tissue, adipose, 313 

areolar, 312 

connective, 294, 310 

elementary, 294, 303 

epithelial, 294, 309 

muscular, 294, 318 

nervous, 294, 321 
Tongue, 25 

attachment of, 26 

base of, 26 

blood-vessels of, 32 

circumvallate papillas of, 336 

dorsum of, 26 

filiform papilte of, 335 

frenum of, 27 

function of, 26 

fungiform papilte of, 336 

glands of, 106, 342 

median raphe of, 27 

mucous membrane of, 335 

muscles of, 28 

muscular tissue of, 349 

nerves of, ^^ 

papillas of, 27 

post-tip, 26 

prebase of, 26 

shape of, 26 

size of, 26 

substance of, 26 
Tonsil, 20 
Tonsillar recess, 20 
Tooth band, 241 

development, cellular stage of, 240, 428 
saccular stage of, 260, 266 

follicle, walls of, 259 

fundamental parts of a, 72 
general description of a, 72 

germs, 242 

roots, preparations for development of, 
282 

sac, 258 

sacs exposed by dissection, 264 



454 



INDEX. 



Tooth sockets, 42 

tissues of a, 72 
Transverse facial artery, 13 

branches of, 13 
vein, 13 
Tunica propria, 365 

Upper bicuspid, first, 133 

occlusal surface, 134 
angles of, 138 
buccal surface of, 136 
calcification of, 133 
crown of, 134 
cusps of, 135 
distal surface of, 138 
measurements of, 133 
mesial surface of, 137 
neck of, 138 
lingual surface of, 137 
pulp cavity of, 206 
roots of, 139 
types of, 140 

second, calcification of, 143 
general description of, 144 
measurements of, 143 
pulp cavities of, 209 
cuspid, 124 

calcification of, 124 

crown, 125 

cusp of, 1 29 

cutting-edge of, 129 

deciduous, 229 

distal surface of, 128 

labial surface of, 125 

measurement of, 124 

mesial surface of, 127 

neck of, 130 

lingual surface of, 126 

pulp cavity of, 204 

root of, 130 

types of, 130 
incisor, central, no 

calcification of, 1 10 
cervical margin, 115 
crown of, in 
cutting-edge of, 114 
developmental grooves, no 
distal surface of, 114 
labial surface of, 111 
measurements of, no 
mesial surface of, 113 
neck of, 115 
lingual surface of, 112 
occlusal surface, 115 
pulp cavity of, 200 
root of, 116 
types of, 116 

lateral, 118 
angles, 122 
calcification of, 118 
crown of, 119 
cutting-edge of, 121 
deciduous, 228 
distal surface of, 121 
labial surface of, 119 



Upper incisor, lateral, lingual surface of, 120 

measurements of, 118 

mesial surface of, 121 
neck of, 122 

pulp cavity of, 203 

root of, 122 

types of, 123 
molar, first, 146 

buccal surface of, 152 

calcification of, 146 

cusps of, 149 

deciduous, 231 

distal surface of, 154 

fossa? and grooves of, 151 

lingual surface of, 153 

marginal ridges of, 147 

measurements of, 146 

mesial surface of, 154 

neck of, 155 

occlusal surface of, 147 

pulp cavity of, 213 

roots of, 155 

types of, 156 
second, 158 

deciduous, 233 

angles of, 164 

buccal surface of, 162 

calcification of, 158 

cusps of, 160 
deciduous, 233 

distal surface of, 163 

fossa? and grooves of, 162 

lingual surface of, 162 

marginal ridges of, 159 

measurement of, 158 

mesial surface of, 163 
neck of, 164 

occlusal surface of, 159 

pulp cavity of, 216 

roots of, 164 
third, 165 

buccal surface of, 167 

calcification of, 165 

cusps of, 169 

distal surface of, 166 

fossa? and grooves of, 170 

lingual surface of, 167 

marginal ridges, 169 

measurements of, 165 

mesial surface of, 166 

occlusal surface of, 168 

pulp cavity of, 216 

types of, 171 
Uvula, 20 

Vein, inferior palatal, 18 

lingual, 33 

superior palatal, 18 
Visceral arches, 411 

furrows, 411 
Voluntary muscular tissue, 320 

Zygomaticus major, 12 
minor, 7